Tangs should never be added to a tank that small. Even the smaller tangs like yellows need a 4ft-long tank minimum. A blue hippo would need on the order of 100gal to accommodate its swimming patterns. Tangs are big, fast swimmers that need loads of space to behave naturally, and they are really easily stressed by being cramped. Adding to the problem, tangs are also incredibly prone to ich and other diseases when stressed (they are more fragile than things like clowns and damsels in that respect).
The inches per gallon thing isn't useful in any tank really. You have to stock based on the behavior of the fish (big swimmer, lives in a hole, etc.), the max size of the fish, and the bioload the tank can handle. Overpopulating a marine tank can cause cascading problems with nutrient export, causing poor water quality, which will in turn cause stress and eventually animal deaths depending on how out of hand it gets. You are pretty well stocked for a 25gal right now; I wouldn't recommend adding any more fish.
