A real basic rule of thumb is 1" fish per 5 gal
in the new aquarium (15") and up to 1" per 3 gal
in an established tank (25"). Using adult lengths, you have 6" of damsels, 6" of clowns, 1 to 3" goby (depends on kind), a 4" coral beauty and 1" of cowfish for a 20" total. That puts you towards the maximum for your tank, maybe 1 more.
You have a cowfish in with Damsels. IDK if I would ever consider that as the cowfish can poison the entire tank if he gets stressed by the mean damsels (and even potentially the clowns).
Cowfish efficeinados please chime in.