Go for it!
I haven't done STS, but similar workouts at the gym and your rotation sounds like it will shock the body. I am not sure about the muscle gains you will make since Circuits and Cardio can work against your increasing mass. but you will lean out and probably see some good inches leave. within a few months of increasing your endurance you will probably be able to do the full 3 month rotation.
I say try it and see what it does for you. If you are looking at getting stronger to eventually get up to the full 90 day STS rotation, I don't think it could hurt.