It pretty much depends on the individual fish. Like Gump says his don't school, whereas I have a mix of 3 different cories (12 cories in total) and they do school together in their own species 95% of the time when sitting still. They often swim about in pairs but when still they tend to keep to their own species.
I think that if there is a large enough group of the one species then they will hang about together. Personally I found my cories much happier and more fun to watch when I introduced more of the same species.