Continuing my last comment: 

Yep, all the pokemon will be available. Kanto pokemon are the most common, then Johto since the two regions share a land border, then various flying and water types of all regions since they migrate, then Hoenn/Sinnoh/Unova/Kalos all at the same rarity amount. The pokemon of different regions will be found in similar places, like Bug/Grass in the forests, Fire/Ground/Fighting/Rock in the mountains, etc. I assume all the ghost types will converge in Lavender Tower in one big Ghost-pokemon rave party.

Cause you know all those ghosties are raving up there. ;)