Hi Shani,
I've just done Pyramid Lower Body for the first time in about 2 months. I have the stability ball & used it for the floor leg work as Cathe did in the tape. I think had I not used the ball the intensity of the work would still be there but not as much.
I think if you can afford to invest in a ball, you should make that investment. You can use the ball to really build your core strength, too. There are "less intense" ways of using the ball to do just that. In fact, I have some back issues & went through physical therapy for 12 weeks last year. Some of the best stuff I did was using the ball. That's really why I made the investment in my ball & am so glad I did.
Hope that helps.