A breeding pair of firemouths would be okay, but two individuals would probably kill each other. to my knowledge even the smallest of polypterus species would be too big. From the words of the youtubes, discus are too large, minimum tank size is a 4 footer, with decent height.
You could do an individual firemouth, some medium sized tetras or barbs, and some medium sized bottom dwellers like cory catfish if you get a sandy substrate.