Fluff-wise it sounds like a pretty cool concept. I like it
Modeling-wise, I'd imagine it'd be nicer for your opponents if you made it distinct somehow ... maybe different basing features, like painting
DA "counts as" models' base edges dark green and
BA "counts as" models' base edges dark red? Also, you may run into some problems with your Mauler Fiend and Hellbrute "counts as" -- remember this is a true
LOS game now. I guess, the models you want to use are actually larger and would thus give you a disadvantage, however. Still, when counts-as models are significantly different from the official ones, you should realize someone might have a problem with it.
Rule-wise, you can't just assume pick up game opponents will be cool with you switching Preferred Enemies out. Any time you do something like that, you need to be up front with your prospective opponent and get their permission. I don't think it will fly at all in almost all tournaments.