The Pokèmon can be introduced retroactively, yes. By that, I mean you can add/edit your roster to introduce any Pokèmon from that region that you think are really cool or that the character might have, add moves that you like, etc. I'm not against referencing the region should that be off importance. I might do any of that myself.
Some of the mechanical things will either remain with that region unless we all agree to add something.

And I don't really have a problem with you joining (again? You never submitted your character or anything). I just ask that you intend to follow through (actually play). xP
And just tell me if you're dropping or are leaving for an extended period of time.

But I want to see the WIP of your character in a PM and actually finished, b!tch. ;D