Actually, doing the SS series "as-is" can lead to overtraining in a bad way. I noticed joint soreness after trying it for about 2 weeks (and I craved heavier upper body work, so I threw in some Cathe upper body once in a while). I suggest doing something similar to the following, and see how it works for you:
Day 1: Slim Series total body workout
Day 2: Firm it up (Ss lower body only)
Day 3: cardio
Day 4: stretching (Cool it off, or yoga, or you could do a short yoga workout more often during the week... I don't think it can hurt)
Day 5: Slim series total body
Day 6: Firm it up
Day 7: Cardio
That's if you usually workout 6 days a week and do stretching one day a week. You might feel more comfortable with a 5- or 6-day schedule.