I'm usually pedantic in terms of correct English, but in this case I make an exception and go for "corys" not "cories", mainly because if I abbreviate a genus name to four letters and then change one of them (and add one, i.e drop the "y" and add "ie") to pluralise, then there's only three matching letters to Corydoras. I'm sure everyone will know what you're talking about in either case.