I had to restart both Y and OR, to get all of them, in one file in AS! Getting the old starters is awesome because they serve such a huge purpose on teams you're using, both in-game and online, and their movesets are so nice once you get to teach them the moves they use the most, like Focus Blast, Ice Beam, Surf, Hydro Pump, Earthquake, etc.
The gen 1 starters are extremely powerful allies, as are the rest of those ones who can both Mega-evolve or not!