Actually it would make no difference if it's a 3000 gallon tank... the bigger, the less of an impact the fish will have on each other. I would assume these fish are in the 500g tank mentioned in your signature? I'm honestly a little surprised they went so aggressively to the purple tang since there are so many of them. My personal experience is that the more tangs, the less they seem to care about new additions. Please do not take that as advice.
The size of the fish and species would be greatly appreciated, along with the size of the purple tang. If they are all relatively similar sized that will help your cause. If you are trying to add a 2" purple tang to an angry mob of 10" bruisers, that won't go so well. Tank size, aquascape, and feeding regimen will also make a big difference in how they behave towards new comers. We won't attack you for your answer, but we need more information in order to give you the best possible advice.
Knowing what we know so far, I'd say up the feeding, use an acclimation box or make your own out of the egg crate, and go from there. If the purple tang is pretty banged up from the first introduction give him time to recover in the refugium (depending on the size of the fuge) before you attempt to add him to the DT again.
I should add that I keep a tang community (about to upgrade to a 520) and I feed them vigorously throughout the day. It helps immensely with aggression. I have a definite pecking order, and my fish get along great for the most part. My Achilles and Fowleri tangs have their occassional sparring match which always ends up in favor of the Achilles, and that's that.
