Well a few faction cards synergize with each other, but other than that the rules give total freedom to mix and match as you wish. I think it will be hard to resist experimenting with different cards, including out of faction cards. Part of the fun is figuring out the strongest combos, after all.
Agreed. I am glad that the vast majority of cards are rather universal.
I was thinking of some 10 10 10 decks to split up the factions.