Are your firemouths a pair? If so they will be likely to be a complete pain in the #16##### flaring to other cichlids, they will not do any damage but will annoy other fish.
You have a good choice of fish to choose from.
Nicaraguans are stunning and a brightly coloured female could work well with your stocking if your wanting a variety of cichlids, Just look at My old one and Wills on the forum to see how stunning they are even a pair but males do get big and bulky so would not leave all that much room for other fish.
Species like sajica/spilirum or honduran red points could work and you would get a pair in there if that is what you are looking for but keep a keen eye at breeding time.
Rainbow cichlids should be a must with a community CA tank a group around 4 would work well and add a lot of colour and decent size without too much aggresion.
Female Salvini could work in there, but they can be hit and miss. Mine was good for a salvini but i would still say that it could of taken out my large ellioti if and when it wanted. Best to get from a very small juvi.
Robertsoni could work.
The problem is most centrals are big and usually more aggresive than the south americans and in a rio 300 there is not all that much to choose from, is it definately centrals you was looking for?
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