you're limited to only other medium sized semi aggressive cichlids for your setup, which adds up to "not much"
I would suggest keyholes, but they're timid and shy, the boisterousness of the firemouths would probably scare them off.
And jewel cichlids are very aggressive, single or paired, only get one if you don't like the rest of your stock. There are reports on fish profiles of them being peaceful online, but I've got to see it to believe it, and I haven't seen it. The jewels at the LFS if they're more than 2 inches long are often tattered and broken from being kept in the same aquarium with each other, which just spells trouble. I'd put them up there with convict cichlids in terms of their aggression. Do yourself a favor, just get more angels.