Ask your PT about using the Shoulder Horn. It is a weightlifting bar, but it totally isolates the shoulder girdle. You cannot cheat with this bar, it helps folks with shoulder/rotator cuff problems.
Just a few quick hits from a former powerlifter:
Never press anything behind your head or neck, always in front.
Always work the entire shoulder girdle: front, rear, and side deltoids.
Do pushing exercises on one day and pulling exercises on a separate day. Chest, Triceps, Shoulders all utilize pushing muscles. Back, Traps, and Arms utilize pulling muslces.
Legs are separate unto themselves.
The Pec, Delt, and Trap tie in is very fragile, so caution should always be used.
Warm up properly, and if you feel any un-natural pain, pull, pinch, etc...stop immediately.