yellow labs are african cichlids, they like hard, alkaline water, the exact opposite of what the rest of your fish like. They're not as boisterous as you'd think, they're just not compatible water-wise. You just don't mix rift valley fish with new world fishes.
Aside from that, firemouths IME really don't like each other. Adding more could be disasterous. IME they're also devils toward other cichlid species. I'd avoid putting any dwarf cichlids with them, my firemouth killed my bolivian rams when he was in his first tank, and was snarky enough to constantly pick on my green terror and severum, twice his size, when I moved him to the 75 gallon.