Oh, I see, thank you. Still, can you terastallyze a pokémon with a mega stone or Z-crystal attached as long as they have not yet used them? That way, you gain flexibility, specially with pokémon like Garchomp or Tyranitar, who are good both mega and non-mega.assuming you mean on the same pokemon, your mistaken, mechanics are limited, meaning a pokemon that can mega cant use a z move / tera, a mon that is using a z move cant mega / tera, and a pokemon that has terastaled cant mega evolve or hold a z move. all of them are generational mechanics so they all take the same spot
EDIT: Reading again, I guess the answer is not. I guess it might make thinks too complicated, and it might not even be easy to program, as someone stated.