Can you choose your planar ally?

[RoboEmperor]

By RAW it says your deity chooses, but can you ask your deity for a specific outsider?
Is there also some alignment restriction?

By RAW it says you can request an individual if you know its name, so can you like scry a generic outsider you want and find its name?

[Belkarseviltwin]

The alignment restriction comes from the fact that Planar Ally spells have alignment descriptors the same as the subtypes of the creature's you're calling- so an LG cleric cannot cast Planar Ally and ask for (or be given) a chaotic or evil creature.

I would say that you can make requests as to what you want- after all, if you can request a specific named creature you should also be able to request a specific species. On the other hand, these are only requests- your deity can choose to send something different.

I would imagine that you can also request a creature that's capable of doing a specific thing- for instance, flying carrying a rider, or casting a specific spell. Again, it's up to your deity (and hence your DM) whether you get that.

[Karnith]

Note that a good number of published deities have designated Planar Allies - see e.g. Complete Divine, in which the deities have a "Herald and Allies" section describing which creatures a god sends as Planar Allies.

[RoboEmperor]

I guess it depends on the DM but I don't really think there is any reason why a Deity would deny a creature request, unless the creature is the enemy of the Deity, despite not being on the allies list. After all, the called creature isn't helping you out of their service to the god, but because you are a potential business partner. The deity merely acts as a telephone wire of the two of you.

Alternatively, you could planar ally an outsider (in this case devil), pay him to teach you the name of some devil you want (not true name), then planar ally that devil.

[Psyren]

By RAW it says your deity chooses, but can you ask your deity for a specific outsider?
Is there also some alignment restriction?

By RAW it says you can request an individual if you know its name, so can you like scry a generic outsider you want and find its name?

By RAW, yes, you can ask for anything you want.

Your DM still decides though.

[Karnith]

Alternatively, you could planar ally an outsider (in this case devil), pay him to teach you the name of some devil you want (not true name), then planar ally that devil.
You could do that, but it doesn't mean that you would get the creature you requested. Per the SRD (http://www.d20srd.org/srd/spells/planarAllyLesser.htm):

If you know an individual creature’s name, you may request that individual by speaking the name during the spell (though you might get a different creature anyway).
(Emphasis mine)

Players/characters simply don't have control over what they get out of Planar Ally, beyond the alignment/deity choice.

[daremetoidareyo]

I guess it depends on the DM but I don't really think there is any reason why a Deity would deny a creature request, unless the creature is the enemy of the Deity, despite not being on the allies list. After all, the called creature isn't helping you out of their service to the god, but because you are a potential business partner. The deity merely acts as a telephone wire of the two of you.

Alternatively, you could planar ally an outsider (in this case devil), pay him to teach you the name of some devil you want (not true name), then planar ally that devil.

This would actually be a pretty neat plot hook. You use planar ally and request the usual, but instead receive something far different, or a little weaker. "I asked for a hound archon, and you gave me a flipping MODRON?!"

The reason: all of the celestials of that type are needed to attend to a huge emergency. Sure, it makes your particular deity look a little weak, but this does two things:1.) reminds PCs that the DM is arbitrator of events (sometimes necessary in many groups) and 2.) sets up a bangin plot hook about inter planar warfare. Maybe the elder creature that IS the entire 37th layer of the abyss woke up and decided to take a big old drink of the waters at the base of mount celestia before eating his morning lesser divine god sausage.

Later, the PC can opt for that modron if he liked him, or his usual planar ally returns with a weird graft instead of his/her usual arm/leg from the battle that they eventually prevailed on.

[Sam K]

There could be some interesting complications with this if you god and the creature you're requesting aren't on the best of terms. For good deities this is likely less of a problem (but good doesn't always mean nice...) but for neutral and evil deities competition could be fierce. Your lawful neutral power may not WANT you in debt to a true neutral outsider who has ties to "the competition". Sort of a "sorry, we have a trade embargo on that plane right now" situation. Alternatively, the outsider you get may have goals that conflicts with those of your deity, which could make for some very interesting bargaining.