Questions :)

[Thelucyfer]

Can I use pokemon from kanto to johto and vice versa after switching kanto and johto?

[Kerbecs]

Once you finish kanto story, and go to johto, all your kanto pokemons will get locked and you'll start johto story with a new starter, low level pokemons. However, once you beat all johto gyms, you will be able to access your kanto pokemons for beating elite 4 or other quests. Also, once you finish johto story, you will be able to use johto pokemons in kanto too. The same goes for hoenn and sinnoh.

In short, for beating 8 gyms of each region, u cant use pokemons from other regions.