I think the size of the tank would also need to be a real consideration. They may be small, but rams still need their space, and the more you can provide them, the less chance of territory issues.
Similar issue with firemouths. Both of this fish will be trying to inhabit the same area/territories in the tank. The larger the tank the more area's to each to claim.
I've never kept firemouths, but have heard they can be quite aggressive. I would say to keep both you might need a 4ft tank or larger. Smaller than this, it would probably be better to stick with just a ram species.