I pretty much have the same size tank as you and I added all of the following at once - 2 yellows (one large, one small), a purple, desjardini, achilles, powder blue, and hippo.
Here was the pecking order....zebrasomas - desjardini (alpha to all fish), small yellow, small purple, large yellow. The powder blue was alpha to the achilles. The PB would occasionally instigate trouble towards the achilles, the achilles would flare its fins and just swim away. The PB never inflicted damage to the achilles. The hippo at 6" and was just happy go lucky.
It only worked for 2 months because then I ran into ich troubles stemming from my hippo tang. I QT'd all my new fish except the existing ones (YT and hippo) and forgot the hippo had broken out with a mild case a couple of years prior (never treated at the time).
I'm freshwater now, and I plan on adding the following once the tank is ready: fowler's, achilles, desjardini, hippo and a purple or yellow (don't have either yet). I love PB tangs, but am hesitant to get another one. It seemed to work well the last time, but you never know with individual specimens. Having 8' really helps if the subordinate fish can just swim away.