I would actually not suggest any starfish at this point in the tanks developement. Stars are extremely sensitive to changes and a new tank will not really be stable for the first few months. For an FOWLR
tank, a star will eventually be possible depending on species but what will you be adding it for? Decoratively or as a detrivore?
I agree with what reefobsessed
stated above, too many fish. The butterfly and tang are rather large growing fish and may be too large for this size tank. I would actually not keep a threadfin in a 55 gal
tank, the tang might be possible if a longer tank- 48". That said though, with a butterfly in any size tank you will not be able to keep corals.