So, in LGPE, there are 153 Master Trainers (1 for each and every Pokémon in the game). Can you battle them with the Alolan Variant instead of the Kantonian or does it ONLY have to be the Kantonian?
yes. the image below shows you can use alolan forms against master trainers: https://imgur.com/gallery/0TwiR6Q
dont believe me? try this: https://www.youtube.com/watch?v=oRxXNxU5WcQ go to 46:43