Hi there,
Congrats on your results! That's great!
I'm not a pro here, but I would say you know best what your body can handle. I personally believe Cathe's rotations are spot on; 3 days of strength + 3 days cardio + 1 day of rest. Any more than that, and you might risk over-training. If you want to add more cardio in addition to your Cathe cardio's, I would recommend walking. You can never take enough steps! Also, walking is a great way to add low impact exercise, burn calories, release lower body tension you might be experiencing from STS legs, and is just plain ol' wonderful for mental clarity and stress release. Very underestimated, but man it works like a charm!
Good luck & take care,
Natasha