I don't know about the positions: my sense is that you should adapt the positions to work with your body, but just be careful not to hurt yourself.
The rule about weight training is that you shouldn't work the same muscle group on consecutive days. The idea is that the recovery period between workouts is the time that your body uses to rebuild and strengthen your muscles and so it is very, very important. But in this case I'd say it probably depends on the intensity of the workouts because I imagine that you could do yoga on consecutive days and you can run on consecutive days. Do these workouts really tire your muscles? Do they feel nice and sore the next day? If the exercise is gentle, you could probably do it every day, BUT, I think the better option would be to do it every other day and add in a different kind of exercise on the intervening days--like walking or jogging or swimming or whatever. Weight training is great, but so is cardio. The combination of the two is a body-changing powerhouse.
Besides, the body likes variety--it keeps it on its toes (literally).