I had them in with severums, convicts, a chocolate, a blue acara and a few others that I can't remember now. I don't know if I just had the incredible luck of picking out four males when I got them at around 1.5". I had them for about 6 months and just got tired of them always fighting amongst themselves when I sold them. I probably would have had better luck if I only kept a single in the tank.
The FM's were probably the smallest fish in the tank when I had them and they really didn't pay attention to anything else that was in the tank although I agree with Matt that the gymnos are not robust enough to keep up with the FMs. I would put the FM's in your 90 and maybe add another species or two to the tank. You could do a pair of cons or cutteri and a single larger fish or two to fill out the tank. If/when your FM's pair up, I would pull the other firemouths out so they don't get harassed to death.