Probably not a good idea to mix most SA dwarfs with CA cichlids. First of all, there is quite a difference in aggression level and ability to defend itself. Firemouths are really aggressive cichlids compared to many SA cichlids, and really could end up bullying most SA apistos.
The term "dwarf cichlid" is an arbitrary term based on the size of the fish. If 4" is the cut off for being designated a "dwarf cichlid", there are no dwarf CA cichlids. There really are no cichlids from CA that won't potentially get at least 5 1/2". Apistos like a cockatoo usually get no more then 3" - 3 1/2". Firemouths can get 6"+. On the internet there are at least 2 pictures showing large males beside a tape measure showing 6 1/2". Supposedly they can get to 7", though 5"- 5 1/2" is probably a more common size for large adults. To put it another way, an apisto at 3"- 3 1/2" would be around a 10 to 15 gram fish. Firemouth would have the potential to exceed 100 grams; with most large adults weighing around 50-80 grams. So it's many times larger of a fish.
Then there is also the aspect of water parameters. FM, like most CA, usually come from very hard, alkaline waters. Many SA, the very opposite, soft and acidic. Of course it varies considerably depending on which species and which waters they come from, but many apistos do come from soft acidic waters. And while specific water parameters are seldom that critical for a fish like FM, they certainly can be for some SA that come from soft, acidic water.
I think if you were going to try and mix FM with a SA dwarf, some kind of dwarf pike would probably fair a lot better. At least it would be able to defend itself, and unlikely to get bullied.
As far as another CA that might coexist with FM (or other thorichthys), less aggressive species form the genera Cryptoheros, Archoecentrus, or Amititlania. Something like HRP (Hondurean Red point) or Flier cichlid (Archoecentrus centrarchus) might be worth a try. Some people have had success mixing rainbow cichlid with other CA, though IME they didn't do well at all with other CA, especially FM, and did much better in a tank with out other cichlids.