I say go for it, I know alot of people are going to shoot me for saying that, but you are not sharing the room in the tank with any other swimmers (chromis dont count). The word of caution is to see which tang is the aggressor, and try to pick a purple a little bigger than that one.
I have seen up to four tang in a 90 live happily ever after. I have three tangs in my 150 for 4 years with miniumal problems, I have 21 fish in all in the tank. 8O Crazy, maybe, but i am enjoying it and there are no problems :!: :wink: