They are cheap.. just like most common cichlids.
They are less aggressive then most Cichlids, but again they are still Cichlids. They will likely beat up on your other fish, especially if they want to mate. I have had luck keeping them with a variety of things, but you need to be careful with your tankmates. I would say they are one of the most 'laid-back' C. American cichlids, and only reach a max size of 6-8".. Actually, I could be wrong on that max size..not positive. But they don't get huge atleast!
Interesting factoid about firemouths.. In the wild they come from very murky waters, and they are TRUE bluffers. If you look at the 'eye-spots' on their gill plates, they serve as a defense mechanism. When a Firemouth encounters another fish in dark, murky waters.. It will flare its bright-red gills, which will then show the eye-spots. This will give the intruding fish the impression there is a very large, aggressive, red Cichlid nearby.. This will usually cause most other fish to retreat.
This is also why Green Terrors have the vibrant green around their face, to ward off intruders in murky waters.
Some fish have eye-spots towards their tail, to confuse predators as to which end is the 'front'..
Hasta,
Miles