Sounds like you have a really nice setup. Salvini and Convicts are much higher up the aggression scale than what you have in there. I think if you add either of them you are risking the peaceful aquarium that you are enjoying. If you must add another cichlid you would be better off with something like a Blue Acara. What about another school of fish or some loaches? There are plenty of other fish you could add besides another cichlid. Good luck.