I would strongly encourage you to pick up Ian Kings Get Buffed books I-III.
If not watch his control drills and abdominal training videos.
Your answers can be found there.
External D.B. rotations are a good control drill and can be a good rotator cuff exercise. I would do these @ the begining of the workout to warm up the shoulder muscles prior workout.
Abdominals train where you feel the need for them? Are they lagging? Do they need priority over other muscles groups? If yes, early in the workout. If no, @ the end? Is your goal abdominal maint., increase strength, speed, definition?
"Failing to plan is planning to fail:.