The reason the tank is too small is agression the best tank mates for red devils are rocks. If you are going to keep two or a breeding pair I would recommend a 90+ gallon tank they like to take as much territory as possible. There is not enough territory in a 75 gallon for two they will fight and the most dominant will kill the other or harrass the female possibly to death. You may get away with putting a divider up but I would opt for a bigger tank or only one red devil.