Personally I don't think a firemouth is the best choice for your tank.You have very soft water and a low ph.This is fine for your severum and a lot of other s/a cichlids.C/a cichlids like firemouths do best in hard alkaline water with a higher ph.
The other potential problem I see with your water is for your existing crayfish.I am no expert on crayfish but I believe they need more calcium in the water to keep their shell healthy.
They are also capable of doing damage to fish when the lights go out.
Severum are normally quite peaceful for their size and don't tend to harass fish they don't feel threatened by.If you never had the crayfish I would suggest a pair of Bolivian rams.Lots of character for a small cichlid.Give them plenty of shelter and they may breed for you.
If you keep the crayfish, then I think your initial plans for blue acaras could work.