I currently have 2 angels (a cb and flame) in a 72 gallon. the coral beauty was in there well over a year before the flame was introduces, and there have been no problems at all. they will be going into a 125 in a month or so, and I may consider a 3rd angel, I don't know.
though I wouldn't recommend it, I have actually seen a 150 gallon with 6 or 7 dwarf angels in it (including THREE flame angels). I don't know what, if any, troubles this guy had, but it's been done.
the fact is, most combinations are gambles in a glass box full of water. there is no trigger that is considered reef safe, but I have read many articles by experts that say a majority of triggers CAN be kept in a full reef tank with no problems. there are always exceptions to every rule. but what I've seen, most of the smaller angels are peaceful for the most part, so careful planning could result in an exception rather than the rule of thumb concerning them.