Tangs are open water swimmers and require large swimming space to prevent stress. Powder browns and powder blues are both VERY easily stressed fish and it is not uncommon for them to break out in ich in even much larger, healthy aquariums, much less a less than ideal environment for them. If you want to keep a tang in a 55, I would recommend a kole tang, yellow tang, or scopus tang.
Others most likely recommended against butterflies, even in a fish only tank, because they are very delicate, very timid, and very difficult to feed. Most do not readily accept prepared food, and many have issues keeping them even when they do. While you may be able to maintain one with religious feeding of vitamin-enriched frozen foods (and quality pellets if you're lucky enough to ween the fish onto them), the ideal environment is a very large tank with large amounts of mature live rock that they can graze on for copepods and other micro-crustaceans, micro-dusters, and algaes.