I say stick w/ the one firemouth & keep you eye on him. Adding multiples of that lil dynamo is asking for trouble. As for tetras, pretty much any as long as they can't fit in the mouth of the cichlids. "T"
+1. Firemouths more bark than bite is not necessarily true, it just varies with the individual (also size) and depends what else you keep them with. If you've got a firemouth that's mellow with your angels that's fine, but doesn't mean adding a few more is a good idea, especially in a 4 ft tank.
That's what fools people sometimes, small fish of a species or new to a tank and they can seem more docile than later on with more size, hormone changes, etc. Not to say yours won't be ok, and starting small being housed with angels in your tank may be a factor, but it's not 100% that what you see now with a smallish juvenile will still be the case later on with more size and adult hormones, something that's true of a number of cichlid species. Not being negative, since things can work out in one tank that might not work for everybody (firemouths with angels often gets mixed reviews), just something to be aware of.