zebrasomas will kick the hell out of acanthurus. Over all that is. Maybe not directly but also by intimidation. The toughest fish are the hippo tang and the sohal.
SO if you keep a tank of tough zebrasomas, I wouldn't put a single acanthurus species in there. But if you went the other way, then a couple of zebrasomas in a population of acanthurus won't hurt.
You've already got 3 aggressive fish. You have to decide what your end number will be. Also, since they are alreay in there, if you add fish, don't just add one at a time, you'll need to dump in 2 or 3 at once, so if you do see the powderblue and just add a little one, I don't think his chances are good, but if you added 3 or 4 (let's say there's a sale, on them) they would form a protective school and have a fighting chance against the hometown advantage. GO TEAM!