Just like Poly and Kueluck said, it's certainly not a good idea to house them together, especially taking into consideration the habitat differences. Southern toads are terrestrial, and naturally burrow, whereas Fire Bellies are semi-aquatic and sit in the water a lot of the time. They aren't from the same environment either and would probably be terrified of each other, and bufotoxins may cause serious damage to the other. Fire bellies are excellent pets though, so if you want some, go for it! Just separate the tanks