My brother has 3 convicts and 2 firemouths in with 1 of my blue acaras they are all about 3" and they seem to get along fine. he also has some tiger barbs in the tank. two of the convicts are currently breeding and kinda terrorize the rest of the tank mates but they all seem fine.
If it works for you, great, but keep in mind that the jewel cichlid - Hemichromis Guttatus is from Africa. Think twice before mixing old world and new world cichlids. I personally don't like the idea.
With enough rock and/or woodwork to allow the establishment of territories, you could add another firemouth or two. I would also add a couple of large and active catfish (synodontis, perhaps) and a small school of silver dollars or rainbowfish to act as target fish and keep the cichlids somewhat distracted from each other.
just in general, sajica, centrarchus, nics, some of the Geo. species will all work just to name a few. lot of fish will work with firemouths and convicts.
this might sound crazy, but how about a smaller green terror. keep the light off for a while while putting it in. this will help with them getting along. you can check up on them. It could be a very aggressive fish but can easily be intimidated. this fish is my favorite because of its color and personality.