So besides S/M, what games are necessary to build a Living Dex? I heard something about only ORAS and XY being necessary, but some confirmation would be nice
Doing the same right now and don't have an answer at the moment but I know ORAS is important for the legendaries.
Yes only Oras and xy are needed. Game Freak even made a point of releasing all event pokemon during ORAs so everything would be available.
I think every legendary is going to be catchable in USM because of the gen VII sign, but right now you need all four gen VI games, yes.