I have kept Salvini's and firemouths together before. It all went well until Salvini's began to breed. Then it was on! Firemouths had to be removed they could not take the abuse.
If you only have one of each it should not be a problem.
I would keep one or the other in that tank. Salvini's can get nasty when they dont have enough terrtiory to call there own. Its up to you the tank could probably handle two but it will all depend on the temperment of your fish. I would do something like a convict salvini combo. I think the convict would hold his own a little better and also stay a little smaller so you dont push the bio load so much.
thats onew of the good things about slavini's there temprement, which means they often work well with smaller cichlids such as convicts and firemouths, but nothing is ever solid mate so keep an eye on them.