It alll depends on the size of the tank. If big enough then go for it! I have kept many zebrasomas in the same tank. As a matter of fact there are 7 yellow tangs in my lfs display that play along great with each other!
If your tank is hmm say 90 gallon+ then your definately ok. If you get the purple then while your acclimating try to move live rock around, moving rock around will make the scopas and yellow less terrritorial. you should be fine!
My 200 consists of >>> 2 yellow tangs, 1 purple tang, 1 achilles tang, 1 sohal tang, 1 moorish idol, 2 blue hippo tangs, 1 powder blue tang, 1 blone naso tang. I put all my fish in at once and everything turned out great. Then again, I take risks. The tang police will be on me soon for sure