I've seen about 6 different kinds of tangs stored together in a 180g. Blue, powderblue, sailfin, purple, yellow, sohal and one more I can't remember specifically. They had ZERO issues since all were added at the same time and all were of relatively similar size.
But regardless, get only one purple, get 3-4 yellow, a sailfin, a regal, and a sohal if you want. You're going to need a lot of nori 8O
As I've said before, tangs are very social creatures (so more is actually better in this case) but they tend to get annoyed with the same tankmates especially if they grow hungry or if added at to the tank at different times after the others have already established territory.
PS. DO NOT PASS UP THE SAILFIN TANG. These things are so neat, they're small, then they're huge... small.. huge.. small... huge.. you get my point.