First off, I'd say that if you are doing the hats in ribbing, it is not so crucial how many stitches you cast on because ribbing is very stretchy, as long as you don't cast on way too many. A finished hat in ribbing will probably fit both a man's and woman's head, again because of the ribs, it will stretch to whatever size is needed.
Take a look at the hat section at www.knittingpatterncentral.com to get an idea of how many stitches get cast on for a hat using the weight of yarn you intend to use and go from there. Also, the free patterns section on this site has some good basic hat patterns that you should be able to adjust.
Doing the decreases without needing dpns, you can use either magic loop knitting or knitting in the round with 2 circular needles. You can find videos for both of these methods here under the heading of small diameter circular knitting.
Swatching in the round, there are some tips in this post: