I agree with the posts above- I think your body will tell you when to say when, and in this case you may want to listen to those who have learned from their own bodies!
Interval Max is a tough one. I feel I'm quite fit, and when I am well rested I can do IM on 10" with no modifications. But I still find it hard to do IM back-to-back with a long run, or with the entire Body Max, or what have you. So I think it's wise to fill the in between days with other workouts.
Lisa