Some of the trigger are OK for reef tank in a sense that they don't pick on the corals and inverterbrates. If you look at the mouth of the triggers, those that have mouth up turn are often OK because they feed off the water column instead of hunting off the reef structure. They amy go after small fish or shrimps especially if these are added after the triggers. Of the triggers the following are known to do OK in reef tank:
Niger, Blue chin, Crosshatch, Sargassum triggers. Niger trigger will get really big, up to 15-18 inches. Triggers are messy eater and will polute a tank more so than other fish of similar size.
Minh