I'm currently working on enemy design for a game, and I'm converting an Obsidian Dragon using the Pathfinder rules on True Dragons. In the description, Obsidian Dragons at the age category I'm going for (7th age, Mature Adult, CR 12) have a Manifester level of 9th, and are treated as Psions of the Metacreativity Discipline.
All good there. For added CR adjustment, I'm adding 1HD of Psion (virtually irrelevant to the question, but it may affect something).
I'm basically treating my dragon as if he were a 10th level manifester with only one level of psion, similarly to prestige classing in the vanican system or advancing an arcane dragon in levels of sorcerer. This means I have base 88 PP/day, and 21 Powers Known.
Would that be the correct way to progress?
Another, very relevant question: How do I select all my powers? Is there a comprehensive list available for all the disciplines? Are there even 21 powers of (metacreativity) that I can select? This limitation on Psions seems to really weaken the class, or I may be missing something (which is more likely).
You've got it incorrect. You actually don't stack in this case, the dragon doesn't manifest as a psion of 7th level. It has a set of SP powers that can be used a certain number of times per day and a ML to let you know they work. Those are two distinct situations.
What you end up having is the dragon, with a +2 will save, +d6 hit points, 2(+mod) PP and 3 first level powers.
You can look up the psion on d20pfsrd.com, you can choose any 3 powers on the general psion and discipline list.
I'm not sure if that's correct.
Gem dragons are dragons who have innate psionic abilities instead of sorcerous abilities. They are stand-offish and loners as a rule, and they are always most concerned about their own needs. Except as noted below, gem dragons adhere to all the dragon rules indicated in the Monster Manual.
Psionic Powers: Even if a particular power noted for a dragon doesn't appear in the Psionics Handbook as a psionic power (instead appearing in the Player's Handbook) the power is considered to be manifested psionically.
Psionics (Sp): The psionic abilities a gem dragon can freely manifest depend on its age and variety. It gains the abilities listed for its age plus all previous ones, using its age category or its psion manifester level (whichever is higher) as the manifester level. The save DC is 10 + the dragon's key ability modifier + power level. All psionic abilities noted for a gem dragon are usable a number of times per day, as noted under each variety.
Emphasis mine.
source
In any case, are Psions really limited to only choosing powers from their own disciplines? According to this list I have more powers known than exist (hypothetically).
I'm not sure if that's correct.
Gem dragons are dragons who have innate psionic abilities instead of sorcerous abilities. They are stand-offish and loners as a rule, and they are always most concerned about their own needs. Except as noted below, gem dragons adhere to all the dragon rules indicated in the Monster Manual.
Psionic Powers: Even if a particular power noted for a dragon doesn't appear in the Psionics Handbook as a psionic power (instead appearing in the Player's Handbook) the power is considered to be manifested psionically.
Psionics (Sp): The psionic abilities a gem dragon can freely manifest depend on its age and variety. It gains the abilities listed for its age plus all previous ones, using its age category or its psion manifester level (whichever is higher) as the manifester level. The save DC is 10 + the dragon's key ability modifier + power level. All psionic abilities noted for a gem dragon are usable a number of times per day, as noted under each variety.
Emphasis mine.
source
Go look at a psion, how the powers are set up and how many they have. Now go look at the page you linked, where there are powers per day, which is exactly NOT how psion's work. They count as manifester of a discipline because that has some fringe benefits for certain powers. They have a ML, so you know how those powers when used X times per day (for duration, damage, range etc) function.
They are not actually Psions however. They have no psion class levels to add your level of psion to, to increase. Unlike let us say an angel who has a cleric level of 14th and chooses spells each day exactly like a cleric would. Said angel would increase their cleric level.
I understand why you can be confused, but you are still incorrect. Just to add to the confusion, looking at it, that dragon might even be 3.0 not 3.5, as it has mention of attack and defense modes. Those were changed into powers in 3.5. The 3.0 set up for Psionics was completely different.
For 3.5 and PFRPG, it works the way I explained. I'm 100% sure it does not work the way you want it to, and 99.9% sure it works the way I say it does (that .1% is because of it being 3.0 and not being the same system as it is now). If you don't believe me, look up Dreamscarred Press Ultimate Psionics, you can see it at d20pfsrd.com as I mentioned earlier.
As per PFRPG rules, to increase a class level the creature needs to have the effective casting of the class, not certain spells X times per day. As I said before two completely different situations and your dragon is the latter one, so it doesn't get to work that way. Everything you need to know is freely available on that site if you want to learn about the current psionic rules. DSP is about as official as you are going to get as Paizo said they will not be doing Psionics as they don't want to introduce a point based spell casting system (not to mention someone already did a bang up job of them if anyone wants to use them). I've already given you the answer, but it would probably help to learn and understand the system so you can figure out why you are incorrect on this.
The Gem Dragons were part of Monster Manual II which was definitely part of the 3.5 run.
I presume spells/day is meant to keep the design simple, but the text flat out calls for Gem Dragons to have psionic manifesting in place of sorcerous casting.
I any case, DM purview allows me to make monsters how I want. It really doesn't matter, I still don't know where a hypothetical psion's powers known list comes from. This is what I would like to know.
The Gem Dragons were part of Monster Manual II which was definitely part of the 3.5 run.
I presume spells/day is meant to keep the design simple, but the text flat out calls for Gem Dragons to have psionic manifesting in place of sorcerous casting.
I any case, DM purview allows me to make monsters how I want. It really doesn't matter, I still don't know where a hypothetical psion's powers known list comes from. This is what I would like to know.
3.5 base game came out in 2003, Expanded Psionics (3.5 version of Psionics) wasn't released until 2004. So if the book came out before that, it obviously wasn't written using the most recent rule set (which had numerous and fundamental changes). I've been a game of Psionics for a very very long time.
Also you are absolutely correct as the GM you can throw together whatever you want... But that isn't why you asked here. You said how it should be done in PFRPG (I read that as it implicated, correctly). And I have explained to you how that is, but the current rules and the reasons. If you want to ignore that, just be aware of what you are doing and that it isn't actually what should happen in our current rule set.
As for how to choose powers, I have explained in a previous post as well as given you a location where the mechanics are listed.
In any case, are Psions really limited to only choosing powers from their own disciplines? According to this list I have more powers known than exist (hypothetically).
If you are building your dragon as a psion, then not only do you get access to the Metacreativity discipline power list, but the Psion/Wilder list as well.
I'm basically treating my dragon as if he were a 10th level manifester with only one level of psion, similarly to prestige classing in the vanican system or advancing an arcane dragon in levels of sorcerer. This means I have base 88 PP/day, and 21 Powers Known.Would that be the correct way to progress?
Skylancer has the right of it. If you are making a 1st level psion that somehow has a Manifester level of 10, then your powers known and power points per day are calculated as a 1st level psion. The manifester level determines the maximum number of power points that you can spend to manifest a power and augment that power in order to "scale" the power's effect.
It sounds like you want to build a 10th level psion in terms of powers known and PP/Day (which would also have a manifester level of 10).
No no no.
I thank you for the input on advancing in Psion levels, but for the sake of parallelism I treat the gem dragon like an arcane dragon, for the spellcasting progressing I use psion in place of Sorcerer.
When advancing a dragon as a Sorcerer, the caster levels stack, but the class features are that of a first level sorcerer (bloodlines and other features).
This is similar to how class features stagnate and spellcasting continues to progress when taking prestige classes.