needlenose if you like gars. I keep one with a rather boisterous young Oscar and they do okay. The O does harass him from time to time. But I think more out of boredom then anything else. I do plan on seperateing the Oscar in the near future as when Full grown a boredom induced strike could do alot more damage to my needlenose. But I don't think a firemouth's relatively smaller size would be an issue. If your firemouth is in general calm for a cichlid I wouldn't see a problem. Just be aware the needlenose full grown at about 12" can and will eat fish 3" or so. Mine took down some sub-adult Brichardi much to my surprize. I would re-arrange the decor just to be on the safe side anyways when introduceing anything into a tank houseing a cichlid. But I would not hesistate to mix the group wyou have with one. as long as nothing was small enough to be eaten by the needlenose. They are not so much aggresive in and of itself. BUt will eat anything that can fit in it's mouth no matter how aggressive it is. though I do agree more Bichir's wouldn't be a bad idea either. Though I'm not sure about the sun/eclipse cat. as long as the bichirs grow faster then it does. cats are stomachs with mouths and the cat is alot like the needlenose. I have both.. in seperate tanks.
