That is very soft water, identical to what I have here in Vancouver.
Livebearers like swordtails and platies are often suggested for tankmates with Firemouths since they all occur in the same geographical area (Mexico), but this needs moderately hard water so I would not go down that road.
With soft water, I would look at some of the larger tetras. Bleeding Heart Tetra (Hyphessobrycon erythrostigma) comes to mind, a group of 7-10.
http://www.seriouslyfish.com/species/hyphessobrycon-erythrostigma/
Lemon Tetra could also work, again a group of 8-10. Another is the Emperor Tetra.
http://www.seriouslyfish.com/species/search/lemon+tetra
http://www.seriouslyfish.com/species/nematobrycon-palmeri/
I've been suggesting fairly robust tetras, adding some varied colour, and not likely to be seen as food. Their activity would probably lessen the Firemouth's shyness once they are all settled. You could also consider a group of
Corydoras catfish (8-12) for the lower level, or one to three Whiptail Catfish (
Rineloricaria parva, which remains smaller and is very peaceful).
http://www.seriouslyfish.com/species/rineloricaria-parva/
Other options might be some of the peaceful barbs. Just avoid any fin nippers like the Tiger, a bad choice with cichlids. There are several possible species.
Byron.