Year ago I bought a sick Purple tang for 10 bucks and put it into 75 gallons reef as the only tang. It was pretty happy I think and fat. After 7 years, all the HLLE healed up nicely. It was the only tang with 2 Lyretail anthias, a pair of Potter angels and 2 percula clowns. It didn't get aggressive as it get older like other people would tell you. I think a 90 gallons is acceptable tank for a small - medium Purple tang BUT it has to be the ONLY tang in it. It was Okay but feel a little bit tight for the fish. Now I got a 120 gallons tank with a single Yellow Tang in it, I feel that the fish is more relax. Its amazing what an extra 6" high and 6" wide can do for a fish. If you have a change to get a bigger tank, do it. But if this is the only tank you have, I think its fine for the Purple Tang
By the way, my rock was very minimal. I only had 4 piece of rock columns about 40 pounds in the tank. I left alot of swimming space for the tang