Advice for an Archer Summoner

Alright, so I am switching characters in a 15th level game, and I want to make a Summoner. I am not looking to totally trick out my Eidolon for offense. I want to ride my Eidolon, and I would like to use Mounted Archery...

Thing is, for the needs of the Campaign I have to be a Gnome or a Halfling, so the obvious racial choice of Elf is out... which is leaving me a little stuck for what to do. As far as I can tell, I have two options:
A) go full Summoner and use a feat for proficiency in Longbow. This way I at least have a full power Eidolon, but I lose a feat getting proficiency (I don't really consider Crossbows to be worth considering, since I couldn't be getting rapid/manyshot working).
B) Take 1 Fighter level. THis will give me proficiency AND another feat, as well as a boost to my Fortitude, but it means being a level behind on spells and Eidolon, and Mosnter Summoning.

I dunno which is better to go with, and was hoping for advice. Also, if there are any feat/evolution suggestions those would be appreciated as well. Thanks!

to save yourself a wasted feat or (the gods forbid!) a class level away from summoner (which is a class that you really do not benefit from multiclassing at all) if you are going to take gnome anyway then take the Master Tinker alternate racial feature and a sprinkling of craft skills, then you are automatically proficient in any weapon you are proficient with, even exotic ones!

If you have access to the Adventurer's Armory, you can take the trait Heirloom Weapon. It gives you a Masterwork weapon of any type at level 1 (you pay the normal cost of the weapon, so 75g for a longbow), and you're proficient with that specific weapon.

The biggest downside is you're only proficient with that one weapon. If it ever gets destroyed, you're out of luck. At least you can get it enchanted, since it's masterwork, though.

The other suggestions are good. I would personally recommend to not lose a level on the summoner. The price is just too high. Rather burn a feat on the weapon proficiency.

I agree with the above, and since you are going to be riding the Eidolon, perhaps you should go with the Serpent type and give it a set of arms and a bow.

If something comes into melee range, it will have it's bite and tail, but usually both of you will be at ranged distance.

You can add extra legs to it for movement until you get Flight, at that point it is cheaper to buy faster flight than to add legs.

Add DEX and STR as an attribute bonus to it. Consider what ranged weapon you want it to be using, using Evo. points is kinda expensive for proficiency, but it is feat starved too so it really is a trade-off.

Of course, you could use the first Eidolon feat to get two Traits and pick Heirloom weapon as one of them.

...perhaps you should go with the Serpent type and give it a set of arms and a bow...You can add extra legs to it for movement ...

Bonus points for ending up with an Eidolon that looks like Trogdor with a bow.

Now all ya gotta do is burninate the countryside and terrorize the peasants.