An online stocking guide is just what it is....a GUIDE. Helpful to get a sense of how to stock a tank, but I personally don't think it should be followed exactly. Fish keeping and stocking isn't quite so black and white, and especially with smaller species there IS a little wiggle room. Plus, each setup is different, a guide like that can't possibly account for everything.
Plus I highly doubt an extra small fish is going to push the tank so dangerously over stocking that its not an option. Fully stocked? Yes. Horribly over stocked? I don't think so. The corys will appreciate the extra friends a little more then the bettas will anyway. So long as regular water changes are maintained and the tank has established its cycle already, I don't see the problem.