I have no experience with multiple PBT. I did see a film on TV where 2 PBT kept guard over a patch of algae. A school of convict tangs descended on them and ate all their algae, despite the best efforts of the PBT's.
The reason I mention this is that I am wondering if PBT prefer to stake out a section of the reef as their own, and are not really a schooling fish per se. If so, then you may be in for trouble trying more than a mated pair in a tank.