I have 4 different types of cory in my tank (1 albino, 1 panda, 1 julii and 2 melanistus) and they do school. They eat together (sometimes in formation lol), swim together, generally hang together and they all sleep/hide in the same spot. However that is just my experience with my fish and all fish are different. I think if you want guaranteed schooling you should go for the same type.