this old B&C thread. Worth taking a look at, for sure.
More, there's really not that much difference between a Traitor Legionnaire and more recent Imperial turncoats. They're Adeptus Astartes in power armor, and are typically armed with bolters. Heck, the biggest difference would be in the way of weaponry, which makes the current CSM book exclusively a Traitor Legions affair. Also, from an in-game perspective, and just because one Legion specialized in a certain thing doesn't give them exclusive rites to that. Sure, the Alpha Legion utilizes cultists, but so do the Word Bearers, and there are innumerable cults dedicated to any one of the Chaos Gods. The Legions also aren't really coherent entities anymore, and have broken into innumerable sects, cults and warbands, all with their own agendas and internal hierarchies. I don't doubt that we'll see some Legion rules in the next Chaos book (if only for the unbelievable screaming from the fanbase if we didn't), but they're not really necessary so much as longed for.