I have yet to encounter any kind of tang that wasn't reef safe. Corals and inverts are fine. Other tangs, on the other hand...this is much less predictable. It will depend a lot on how large your tank is, as well as which other tangs you wish to keep. Powder Blues are in the Acanthurus group, which includes all powder types (brown, blue, gold rim, Achilles, etc), Orange Shoulder, Chocolate (mimic). No tangs are generally recommended for mixing with other tangs, especially with other tangs with similarly shaped bodies.
That being said, I have 5 tangs in a 300 gallon system...a yellow, a Desjardini (both Zebrasoma family), an Orange Shoulder, a half black mimic (both Acanthurus), and a blue (Dory). All coexist pretty well. Lots of swimming room, lots of rocks, and the smaller "guys" were added first. Powder Blues are notorious for aggression, especially after they've grown...so if you decide to go with one, it should be added to the system last.
Also, all tangs have thinner slime coats than many other fish species, and are much more susceptible to parasitic infections (ich), powder types being the most susceptible of all. Make sure to quarantine any new additions to give them the best chance of success. And keep them well fed...less hunger often leads to less competition, which can help with aggression.
