Normally having a group of tangs will disperse aggression enough that no one fish will be injured. They will still spar with each other though. It is very much like keeping african cichlids where it is very difficult to keep only a few together because the dominant fish will kill the others but if you have a larger number of them things become peaceful, well relatively peaceful. With tangs there are exceptions and you could end up with one that becomes hyper dominant and will do it's best to kill off the others. In that case, the best solution is to get rid of the hyper dominant fish. Of course, to successfully keep a group of tangs you have to have sufficient space and excellent filtration to maintian high water quality.

Some things that help are to add them in groups. Adding them one at a time makes for a very rough introduction for the newcomer. Also if your planning a mixed group of tangs you'll want to add the more peaceful species first and let them get established before adding the more aggressive species such as Powder Blue's, Purple's etc.

Then keep your fingers crossed. There's a bit of luck involved too.