You are correct, 50 gallons will be too small for a pair ofTerrors
It may also be too small for a group of firemouths when they are fully grown, especially if there is also a group of raphael catfish and dithers. But if you get small ones you should be OK for a year or two. Not the fastest growing cichlids. Eventually, a group of 5 FM's will need something more like a 75 + in my opinion.