I only have experience with Yellows so I have no first hand knowledge to offer, but general advice is to look at the Full Grown size of the fish you have and the fish you're considering. When purchasing fish you need to look ahead and make sure you won't have to give them up if they out grow your tank. For example, 5 or 6 fish that are 4 or 5 inches in size would be just fine.... but when they grow up, if they get to be 6 or 7 inches each you then have 6, 7-inch fish in your tank and things suddenly become crowded.
Aside from size issues, be sure you check agressiveness to be sure they're compatible. Just because they're all in the "Tang" family doesn't necessarly mean they'll get along.