Urgh situps, such a horrible exercise. Axler, C. and McGill, S. from the University of Waterloo would agree.
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Give that a read. Situps places excessive stress on the lumbar region of your spin and with prolonged exposure to such activities will bring forth the onset of chronic lower back pain. Alternative exercises, such as crunches, place less stress on the lumbar and will provide comparable levels of stress to the abdominal areas.
Personally I do love to use body weight training programs. For the last 4 months I've been doing them, shifting away from a power lifting circuit. While your combo set does provide adequate challenge, there are undo risks with it that can be easily avoided. Instead of doing a pushup, situp, squat rotation you can simply break it up into two sets. One set where you simple do a burpee (push up to a jump squat) to target your pectorals, triceps, and legs. Then the second set could target different muscle grouping, including your abdominal. This way you target each muscle group maximally while minimizing undo risk. A body weight ab exercise I like to do would involve doing a chin up and then during the eccentric phase you do a leg lift. This way you are targeting your biceps, lats, as well as your abs.
Even then, personally I stick to single exercises when I do my body weight training. Most recently my workout consisted on a 2 sets of 25 reps regiment. It was: body weight rows, crunches, pushups, 24" box jumps, and leg lifts using a dip stand. I would do one set of 25 reps for each exercise going as fast as possible while maintaining proper form and then cycle back for a second set. Progress was measured by the reduction in time it took to complete.
