The species profile on this forum says a pair can live in 30 gallons. I don't see why 1 of them will need more space then...
30 gal is often recommended as the minimum size for a pair but if you've ever seen the size of a mature fully grown pair you might wonder about that. Anyway I think a BN pleco would be a good tankmate.
IMO, a group of large Tetras would be suitable tankmates for a solo FM. Columbian, Bleeding Heart & Buenos Aires Tetras eventually grow bigger than bite size.
It's been my experience that FM males grow to about six inches & become rather thick. Females top out between four to five inches.
Eventually a 29 gal. might be a little tight.
I kept a breeding pair in a 29 gallon for a few months. Outside of that, I don't know that I'd recommend it. They get big, bulky, and are pretty mean if you have a pair.
If you're sold on it, I'd either just keep one and some swordtails/platies/small school or larger tetras, or a pair.
They love to dig, so make sure you have substrate that they can fit in their mouths.
Hmm, okay. I may get about 6 of one of those species and maybe 1 bristle nose pleco if I have some glass algae. I don't think the other guys are really a problem if I keep up with water changes because they are small, but yeah I feel like there is a possibility that I may have to rehome the fire mouth if he gets too large. Thanks for the replies, everyone!