actually powder blues should be one of the last tangs added to a setup. they are one of the most aggressive but it also depends on size. i had 7 tangs at one point sailfin lrg, powder blue med, 2 yellow tang med, blonde naso med, and 2 blue hippo med, sm. depending on the order you put them in they will get along. you have to add them based on there aggression and size.
1. naso
2. hippo/yellow
3. powder blue
4. sailfin
also the size of your tank plays a huge factor. and of course the bigger the better so that tangs can establish their territories (reason for aggression)
this was my experience and would be different for everyone. but best advise is research the fish you plan on purchasing before you add it