How big is the tank? This may be a little risky... Severum should be okay, if you have a big enough tank, but I don't know about the Firemouth. Once the Firemouth reaches maturity it should get a little aggressive and beat up on the angel...
The tank size is 180 litres, the firemouths are still only small,about 3inch, if problems occour i can always take the fish back, should i give it a go ?