Your right about the Rams in that the water might be different, they might be OK size wise.
I was originally thinking of a number of fish (not all together in the tank). Firstly, Agassizi's dwarf cichlid Apistogramma agassizii which would work if your Firemouths are of the peaceful kind (some are nasty and some are fine). Also Keyhole Cichlids Cleithracara maronii which grow bigger than Apistos, about 4". I was also thinking if it would be possible to pull off Kribs Pelvicachromis pulcher as they are much more similar in temperament though they are a risk to your Firemouths if/when they breed and likewise with the Firemouths being aggro to the Kribs. They are all common.