That sounds like a fair to middlin' tank. On the guppies: Unless you can steel yourself to euthanize the fry on a fairly regular basis (or have a spare tank to grow them out and return them to the LFS), stick with all males or all females. I've got guppies in 6 tanks at the moment - 3 cycling, 1 display, 2 where fry put in as feeders escaped long enough to grow. In the display tank and two of the cycling tanks, it's all males. They get along just fine. So, in a 15 gallon, a half dozen nice males would be fine (or get a male and a pair of females, let them breed once, return the females to the LFS, grow out the fry, and keep the best 6 males).
As for the cory, while I myself would not keep a single cory, there are those here who would argue that it is not ecessary to keep them in a group. Some cories kept singly will get used to it and thrive. Some won't. You won't get natural behavior, but with fish from Southeast Asia, Trinidad, and the Amazon Basin, you're not going to get natural behavior anyway.