How about ruby nerite snails, zebra nerite snails, or horned nerite snails? (sorry, got carried away) I'd also recommend a bristlenose pleco or clown pleco. I'd like to see your firemouth try and eat one of those.
Bristlenose would get my vote for sure. I have seen a Firemouth try to eat a small BN. It didn't go well for the Firemouth. A couple of spines in the side of the mouth & it soon decided the BN was not edible.