my lfs has a purple tang and two yellow tangs in the show tank with no problem not all fish have the same temperment so usually when people quote something its not because they have done it but because they have read it
I have a yellow tang and a hippo tang together they were each in their own tanks until I upgradded to a larger tank and put them together. They never bother eachother. If one was established first then another added it could could have been a different story.
Ditto !
I added a Kole Tang to my tank with a Purple Tang that had already been established for around 2-3 years. Although there was definitely initial aggression I made sure the Kole was considerably bigger and robust so he could withstand the onslaught.