Phi-Bi

Technically no but it is very common. Most of the time there is no reason to not to but I don't think people will mind if it isn't. 

Just thinking but I wouldn't mind if it would be impossible to catch dark and ghost types if you play as an apprentice of a christian cult. Or if you make a story about a gym leader to not be able to catch pkmn outside its type(s).

Good to hear. Actually, the reason is technical since we intend to use high resolution sprites, it'd be impossible to sprite all pokemon with limited time. Not to mention that it'd be awkward to include non-story relevant mascot legends...