Red Devils are too agressiv, I'd stick to something along the lines of large-ish SA (chocolates, geos) or small CA cichlids (rainbow, Cryptoheros) of lesser aggression.
Don't see the problem adding an oscar as long as the filtration and WC schedule is good. Otherwise, smaller species like Honduran red points would be interesting.
If you don't want pairs, you might be able to get away with a blue acara and a firemouth in that tank, but there's no guarantees considering the individual personalities and the small space involved.
Neither the severum or oscar would fit in this tank under any circumstances.
I've found them to be more aggressive than usually stated by "species profiles", but only in that they tend to defend themselves well against larger fish.
the flowehorn will probably end up killing most of the fish in that list, especially in a tank that size. a flowerhorn alone would need the whole tank, and with big fish like the dempsey and oscar, you shouldn't be adding anything anyway.
Don't add a convict or salvini, as they are far too aggressive to go with a firemouth in a tank that small, and the salvini gets a bit too large as well. The blood parrrot gets far too large for the OP's tank.