I would caution against more than one Gramma species in such a small tank, ordinarily they don't toletate conspecifics well, though if small and lots of cave structures they might be OK. Firefish usually harem as Dave mentioned, the males fight amongst them selves. This is frequent with Serranidae, Pseudochromis, anthias etc.
With Pomacentrids, i.e. clowns and damsels the females are noticably larger and courtships can be brutal. With most of the
dwarf angels(Centropyge) they start off female and one will turn male if they become a breeding pair