I"m quite surprised that I have been able to pull it off, but I have had a small firemouth (1 inch), a juvenile female convict (2.5 inches) and a fully grown rainbow male (4.5 inches) living happily in a 40 breeder for a few weeks now. A month ago the rainbow would beat up the convict regularly to the point where I put in a divider in off and on for a couple of weeks. It gave the convict a break. Over time the convict has staked out a space in the tank and is able to defend its territory and the rainbow appears to respect that. The firemouth and the convict get along surprisingly well (zero conflict so far) and the rainbow will occasionally bother the firemouth, but nothing serious. At one point there was so much conflict I didn't think this setup would work, but I added a large piece of wood and some plants and re-arranged my rocks. So far so good.