I have a 150 gallon tank with 3 yellow tangs, 1 blue hippo tang, and a sohol tang. There will always be a certain level of stess when new tank mates are added, just try to make to intro as calm as possible, after a feeding or just before lights out or even after a water change. ( not calm situtations I know but they are distractions to the tank)
I plan on adding a purple tang and maybe even a blonde naso in the future i have not decided as of yet, but in a 90 you should be ok