I'm going to offer the completely opposite opinion here. The corys will shoal together, but only out of necessity, not by choice. Visual cues, like color and shape, are the primary determiners in shoaling behavior. If the fish aren't the same color, their natural instinct will be to not shoal together. But, if they are stressed or otherwise feel threatened, it is better to shoal will fish that don't look alike than to not shoal at all. So, they will do it. But, the fish will be far, far, far more comfortable shoaling with fish that look like themselves. To do otherwise would go against the fish's natural instinct. And, really, replicating a little slice of nature is my personal goal in keeping an aquarium.