I would like to complete the dex so that I can get a shiny charm. Well, I was wondering what happens if you put one of the newer Pokémon into the X and Y games. I though that the game will be messed up because of unknown data. Can anyone help me? Thanks!
It will not let you. You need pokebank to transfer mons into Sun and Moon in the first place, and you can't send sun moon mons back into XY.