>>28581105They can do whatever they want on the condition that they complete all the Eeveelutions first.
GameFreak themselves probably are aware that it wouldn't make sense for existing Eeveelutions to get Megas, yet have the base Eevee, which is supposed to be the most reactive form of the entire group, remain un-adaptive to the remaining types.
Regional forms, though, that's a stickier question. Technically there's nothing wrong with the high-adaptability Pokemon having a regional form for each separate evolution.
