billyzbear, a pair of bimacs would probably be o.k. for the 150. They may end up being the most dominant species of anthias in the tank though. They are roughly the size of squarebacks with the aggresiveness of a lyretail. dispars will likely be chased but not killed by the male bimac (given you have plenty of swim throughs and hiding places.) this is what mine does to the six dispars in my system. i haven't had bicolors yet. he doesn't show interest in any other anthias besides the dispars for some reason.
I have over 50 fish in my system. If i listed them all, I'd have all kinds of fish police making nasty comments that I'd rather avoid. Let's just say that the system is primarily stocked with tangs, anthias, and wrasse (fairy and other reef safe types). Then there are all kinds of gobies and other docile reef safe fish in between. The issue on the Sohal Tang has been around for while. Of 20 or so Sohal owners that I know of, only 10% of the ones kept alive long enough have had an opportunity to see that type of aggression. you don't hear a lot from the other 90% because they haven't had any problems yet. I'm still in the other 90% so I can't really tell you from experience.
hope you're able to find a nice pair of bimacs, It's been a while since I've seen any.