purples i've found are soooo insanely aggressive its amazing, not just to other tangs. i probably wouldnt try adding a pbt....i have a 5foot 90gal, and i got the purple when he was size of a quarter, added a foxface that was atleast double the size of the purple he tried to bully the foxface, until the morning i noticed lots of white spots on the purple (the foxface must of stuck the purple with his spines) after that they were good...fast forward 6months later added a flame angel, only reason the flame survived was he could hide in some smaller coves....then another 3months after that, added a lawnmower blenny...i woke up in the morning and was shocked to see the purple beating up the blenny! kept batting and tail slicing the blenny....that was 15$ wasted...
so just saying that pbt's are ich magnets to begin with supposedly, a purple will just make it worse...lol only fish i'd wanna introduce to a established purple tang is either a clown tang or sohal..ha...(the only 2 meaner fish)