Re: External rotation
You don't need the cable rotations. If you get to the point that you need isolation for the infraspinatus, you already messed up.
BUT, I think everyone should be doing shoulder prehab. The best movements:
- Band Pull Aparts
- Face Pulls (neutral grip with external rotation)
All of these have one thing in common, and it's that they work the external rotators of the shoulder, namely the Posterior Deltoid, the Infraspinatus, and the Teres Minor. They also encourage scapular retraction, which protects the shoulder during complex movement, and they balance the internal rotation that tends to dominate in lifters of all types. The Dislocates stretch all of the internal rotators of the shoulder.
Get some light resistance bands (like, light rehab bands) and do these to finish off every workout - right there you fulfilled your prehab requirements. Do them for high reps, go for the pump, go for blood flow, do them AFTER any pressing or jerking (or anything else) so you don't fatigue the stabilizers before strenuous activity.
Also, like Justin is talking about, be mindful of your shoulder orientation during normal big compound movements.