Relic Castle

I have read http://pokemonessentials.wikia.com/wiki/Forms, and I am still confused on how form evolutions work. Say I have a Pikachu, and I am unsure if I would like to evolve it into a Kantonian or an Alolan Raichu. Would it be possible to have the option of Pikachu evolving into Raichu by Thunder Stone, and Alolan Raichu with a separate item? Thanks.