First off, what species of Fire Belly Newt do you have? Many species are labeled "Fire Belly" such as Cynops orientalis, Pachytriton labiatus, and some Paramesotritons. The Pachytritons and Paramesotritons can be very territorial, even to members of their same species. Its not unheard of for them to cause life threatening injuries to others, or even straight out kill tank mates.
Cynops species on the other hand are much more docile. They do well in same species tanks. That being said, however, I would never recommend mixing species. Though most Cynops have the same housing requirements, there can be enough of a size discrepancy across species that the smaller animals may feel bullied or threatened just by the presence of a larger animal, even if the larger animal shows no signs of aggression. I myself keep both Cynops orientalis and C. pyrrhogaster ("Japanese Fire Belly) and the adults of both species are very different in size. I am sure there are others who have mixed these species with no problems and the animals did fine for years and years, but I am a bit of a purest and really dislike the idea of a mixed species newt tank. Neither Co's or Cp's are difficult to find captive bred (try looking in the For Sale section) that if you would like to add animals to a current tank it should not be too difficult to find the right species.
Heather
