Ok, heres the deal.
Firemouth is not a super aggressive fish. It doesn't have a huge mouth like a Jaguar or an Oscar. It doesn't have a habit of eating all other fish.
However, it can be aggressive as im sure you know. If you have fish that are bothersome to the Firemouth, you can expect that at some point they will die. This doesnt only mean that they nip or bite him, but they take his space. In that size tank the FM might feel invaded, and adding more small fish has both pros and cons.
Adding more means more targets to distribute aggression, so each individual fish takes less damage from an attacker.
However, it increases the crowded feeling and adds bioload. Both are unhealthy things in most circumstances. (Bioload always is, overcrowding can be good.)
So, i would personally either do a bunch of either Cories or Serpae. Or do a school of only about 8 Serpae and like 4 Cories and get rid of the Pleco, as it could be territorial to the Cories and is also a high producer of bioload.
However, adding in a school of Serpae, Cories, and an FM with the Bristlenose CAN be done. I stress "can". Its not a great chance when the FM and Pleco are full grown.
But, in the end its up to you.
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