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the rules on cargo bays do state a maximum size that can fit in them, Large.

So any large or smaller vehicle (such as an exploration buggy, which is large) should fit inside a cargo bay.

Unfortunatly they dont list the weights of vehicles (as far as I can find?) so I dont know how much room youd have left over, since cargo bay capacity is measured in tons

(it also says that 4 cargo holds can fit a huge object, and 8 can fit a gargantuan)

since biohacks are not syringes though, this doesnt apply to biohacks. at no point does the ability say that you create a syringe or ammo to be fired, you simply apply the biohack to 'the attack' not to the gun, not to the ammo, to 'the attack'.
the ability even implies this by saying that you dont need to 'pre-load' your weapon with biohacks, thats because they arnt ammo (or syringes), they are just an effect that happens because 'science!'.

If you are looking to wrap your head around how this works in a logical way, the best I can think of is that your microlab has a small teleporter inside it and can 'lock on' to ammo fired by your weapons (as long as they 'inject' into the target, ie, have the inject property) and then teleport the compounds directly into your targets blood stream (or similar). this would also explain why you dont need to roll to hit when targeting someone linked to your microlab, its already got a lock on them and can just teleport stuff into them like a transporter from startrek, heh.
Of course the biohack description doesn't say anything like this, but its the best ive got for verisimilitude :P

because the rules say so.

I understand why you are making the assumption, it makes sense, but im going explicitly by the rules here, and they simply say that the weapon needs to have the injection property, not that they need to have that property because its being injected instead of the normal ammo.

the whole description of biohacks implies that the biohack is being applied in addition to the attacks normal effects, it does not say anywhere that it replaces the effects, or that it is replacing the ammo you are using, it even says that the biohack is expended in addition to the ammo itself, implying that the biohack is applied in addition to the burny acid effect. (the acid being the ammo itself).

note that the fluff discription of the disintigrator weapons (which all the causto- weapons are, and only the cuasto- weapons and injection glove are) says that they dont actually inject acid, and generate an energy field that breaks down the bonds between atoms, creating an effect similar to an acid. how does this energy field then allow you to inject them with poison instead of the acid like effect? no idea, the causto- weapons are a bit 'wierd science' :D but then so are biohackers *shrug*

its also worth noting that the caustolance does less damage than equivalent weapons, the tier 1 laser rifle does 1d8 damage and at double the range, for example.
I believe this is to balance them with biohackers using biohacks in mind.

Heh, as I have mentioned in other threads...

I think the mistake people are making is the logical assumption that biohacks are some sort of ammo

'Syringes can be loaded into these weapons, allowing other materials to be quickly injected instead of an acidic discharge (in which case only the injection effect occurs—no acid damage is dealt'

biohacks are not syringes, they are not a material that gets injected, they are an EX ability that is applied to the attack, they do not replace the normal effect of the attack, they are in addition to it, the above quoted rule does not apply to biohacks, only to other substances you might want to inject with the weapons (such as poisons).

it seems clear to me that the causto- weapons and syringe sticks etc were added specifically as an option for biohackers, since they appeared in the same book and the core rulebook has very few injection weapons (maybe none?)
infact I wouldnt be surprised if they invented the causto- weapons specifically as a way of applying biohacks to targets with high DR, since they are the only weapons (along with the injection glove) to deal energy damage instead of piercing. Without them biohackers would get kinda screwed over by high DR enemies

From reading over the rules...

1. Do the caustolance do damage when injecting something? The text seems to indicate that it works as the caustoject, but I'm not sure about that.

RAW yes it still does damage, but RAI id assume not, as someone else said, best to ask your GM

2. Can I inject a poison and a biohack at the same time with an injection weapon? The text of the Biohack class feature says "any attack" so I supose it works? (I ask this question because I think it's relevant to the next question)

yes, a biohack is an EX ability that adds an extra effect on top of the attack its boosting, it is NOT ammo, and it is NOT somthing that fills the syringe or whatever, it is in addition to the attacks normal effect, you can use any ammo type with it, including drugs, poisons, and medicinals.

Note however, that the attack MUST DEAL DAMAGE in order to apply an inhibitor, so if you use a biohack with a drug for example, it will fail to apply the inhibitor as it will fail to deal damage (at least in the case of the caustoject, caustolance is down to GM, as above)

3. Do the caustolance and caustoject do damage when injecting a biohack without injecting something else? Personally, I would say yes if the answer of #2 is yes since it would means that the biohack doesn't count as something using the injection property of the weapon, thus not disabling the damage.

yes, they not only still deal damage, they HAVE to in order to apply the inhibitor, otherwise theyd be completely useless to biohackers.

4. Same as #2, but with medicinals made with the Medication Mastery theorem.

5. Same as #3, but with medicinals made with the Medication Mastery theorem.

Medication Mastery explicitly states it cant be added with a biohack, but it should still deal damage for the same reason that biohacks still deal damage, medication mastery applies its effect in addition to the attack, its not actually replacing the ammo, I know fluff wise youd assume it does, but rules wise it does not state that it is ammo of any kind, just that it is added to the attack (ie, is in addition to the attacks normal effect)

6. Can I use the Medication Mastery theorem with an attack that use a medicinal not made with the theorem? The idea is to use two medicinals at the same time.

techinically yes, for the same reason as above, it is in addition to the attack, and if the attack is using ammo filled with a medicinal, then it is in additional to that ammo.

7. Why Spark Of Ingenuity can't be used with biohack given by Fields of Study?

the wording says only 'basic' or 'minor' biohacks, and its not clear is fields of study biohack are 'basic' or not.

However, the wording also mentions about if they have different durations, and all 'basic' and 'minor' biohacks have the same durations... so this implies that the fields of study may count as basic? ask your GM :)
PS: on reading the rules for basic biohacks, it implies that basic biohacks can have different durations from the default, but none of the ones listed imediatly under it do, some fields of study biohacks do however, this I would say is again implying that fields of study are also 'basic' biohacks.

8. Can I load multiple substances in the caustoject in advance?

I believe so yes, though only one substance fired per shot (not including biohacks and medication mastery)

9. Same as #8, but with the others injections weapons in general.

again I would assume yes, remember that injection expert says you can even choose which substance is fired next regardless of the order in which you loaded the ammo.

('When you fire a weapon with the injection special property and you have multiple types of ammunition loaded into the weapon, you choose
which type of ammunition you fire when you attempt the attack')

10. Do the conserving weapon fusion prevent the waste of a biohack when missing a range attack?

This I would say is down to your GM, since the biohack is NOT ammo, but an added effect on to the attack, I would assume it DOESN'T prevent the biohack from being wasted, your GM may dissagree however.

11. The glove needler is a small arms that has a range of "reach ft". Do I make range attack rolls with it? Does it provoke attacks of opportunity?

read its description in the book ;) it explicitly states that it doesnt provoke dispite being a ranged attack at melee range.

note that you could also use a syringe stick, as they are operative weapons in addition to injection weapons, which means they get to add DEX instead of STR just like a ranged attack.

12. What happens if I try to use a biohack with a blast attack as the one given by the blasting weapon fusion?

Oh wow, I didnt think of that, I think thats down to GM.

both the initial declaration of a blast attack, as well as each individual roll against each target is worded as 'an attack' and biohacks apply to 'an attack' so I could see it being ruled either as effecting eveyone hit, or only effecting the first target in the cone that gets hit (since you roll for each target one at a time form closest out)
note that the wording on biohacks also says that its the action used to make the attack that is applying the biohack (not the ammo itself, even though it goes on to a say that misses mean the biohack isnt applied)
I can see this being OP though, so id probably want to rule it as only applying to the first target hit.

13. Same as #12, but with poisons, drugs, medicinals, serums, etc.

I believe that the relevent ammo should apply to ALL targets hit by the blast, as the wording on blast explicitly says that ammo for blast weapons is designed to do that, and it only expends the one ammo.

Seems pretty strong, but its harder to argue against this one, note to self: get blast on inject weapons!

14. Can I stack the effect of a basic inhibitor and the effect of a unique inhibitor given by a field of study? Personally, I would say yes since it is only stated that the basic inhibitor can't stack with itself and that the minor inhibitor can't stack with the basic inhibitor, but I want to be sure...

based on my answer to 7, I would assume that field of study biohacks are 'basic' biohacks and as such no biohacks can be stacked with any other biohacks.

15. Is there any way to make the poisons less expensive?

not that im aware of, unless your GM houserules somthing

16. Can I use poisons with explosive ammo?

Id assume no, since using a poison is using a specific type of ammo, you cant use more than one ammo at the same time :P

you can have explosive ammo and poison ammo loaded in the gun at the same time though, and due to injection expert, can pick which one is fired each time regardless of load order.

This is all based on my readings of the rules of course, but im pretty confident on these answers

(EDIT: formatting)

I missed somthing in my last post, the last paragraph should state that it would also cost 1 biohack useage as well as the 2 charges and 1 standard action, heh.

But to make it clearer, the order in which things would happen, based on how the rules are written, should be as follows:

1: biohacker declares they wish to make an attack action (or a double-tap attack action, or any other applicable attack action).
2: biohacker declares that they will use a caustoject, which has the inject property.
3: biohacker declares they wish to use a biohack with this attack action. They should also declare which biohack (I will assume its an inhibitor).
4: biohacker rolls to hit (applying all relevent modifiers)
5: if it misses, the biohacker loses the appropriate amount of ammo and 1 use of their biohack ability, these are wasted and the attack action ends.
6: if it hits, the biohacker rolls damage.
7: damage is resolved, taking into account DR, resistance, or any other applicable effect/ability
8: if the damage is reduced to 0, the ammo is expended and the biohacker loses 1 use of the biohack ability, these are wasted as no damage is dealt and no inhibitor is applied.
9: if at least 1 point of damage is dealt, the inhibitor is now applied as per its rules, the biohacker loses the relevent ammo and 1 use of the biohack ability, this is obviosuly not wasted as damage was dealt and the biohack took effect (unless the target saves, assuming the biohack allows a save, most dont!!)

note that this means that if you apply an inhibitor to lower the targets acid resistance, this lowering of resistance WILL NOT affect the attack that applies the biohack, as damage must be resolved BEFORE the inhibitor is applied. (it will of course affect subsequent attacks, assuming it actually managed to deal damage past their resistance)

all of this is assuming you want to use your weapon with the biohack (and are using it on an enemy), biohacks can also be injected directly without the use of a weapon as described in the biohack ability rules, as well as be used on allies with boosters etc.

actually the caustoject and caustolance will still get to deal their acid damage IN ADDITION to a biohack

a biohack IS NOT a syringe or other compound you are injecting (in rules terms) Biohack is an EX ability, an extraordinary ability that augments an attack made with any weapon with the injection ability. Biohack specifically states that the attack that is being augmented with an inhibitor must successfully deal damage, heavily implying that it always augments an attack in addition to its base damage.

That last bit of text on the caustoject description is in refferance to any character (biohacker or not) that uses it to deliver some other compound, such as a drug they perchased from a shop or somthing.

Thus, a double-tap attack with a casutoject wielded by a biohacker using a biohack will get +1 to hit, deal its base acid damage + full weapon specialisation damage (+any other applicable modifiers), and apply the inhibitor, all at the cost of 2 charges and 1 standard action