I believe that most cowfish need a 100 gallon tank or larger since some specimens can reach 16-20 inches. They are difficult to keep and when they are stressed they release a toxin from a gland in their skin that is leathal to other fish. The damsels would probably stress him out. I'm not an expert, but I dont think that would be a good fish to add. Check out what Dr Fosters and Smith has to say about them.
Go Big Blue!!