If the serpae tetras are in a large enough group, they should (in theory) keep their aggression to their own school members. You need a group of 8+ for this though, not just 4. Also, the khulis will hide most of the time and you'll never see them in that size tank unless kept in a large group (again, 8+).
For the Rams, GBRs and gold rams are two color morphs of the same species, so they have equal aggression levels. You do have space for 2 pairs, but make sure you add both pairs at the same time or else they'll fight until the other pair is dead. I've never kept firemouths so I can't comment on their aggression.