Severums and Labs are going to be ok with Hatchets and many other shoalers, but you'll want to watch out for the firemouths - they're a little unpredictable. Sometimes they get along well with shoaling non-cichlids, other times they will harass them. Basically it's going to be a bit of a risk either way. You could try jumbo danio's, dwarf Rainbows, or one of a number of different rasbora's. Something quick is best.
As it stands now, in a tank that big and with the species you've chosen, the mix will probably work out, but I hope you realize the behavioral and dietary differences between African Rift lake fish and New worlds when you start mixing them or you could end up with a disaster on your hands. Water parameters are usually more of a problem in theory then reality - both types of fish can handle typical neutral to slightly alkaline tap water.