I'm skinny too, and just started doing a 2 times a week total body workout about 3 weeks ago. I hope that it helps; can't tell yet! All of the advice I read here has said to try two times a week for each body part.
You can do do full body workouts by shortening the PS workouts or the CTX workouts. The MIS would get boring to do two times a week for a more than a couple of weeks!
Sandi M