Here's the truth.
It is impossible to isolate the small muscles. There is no literature to support that notion. Moreover, the idea that strengthening the shoulder as a whole will not contribute to strength of the rotator cuff in general is wrong.
Finally, there's a huge issue with the word 'strength' in the rehab field. When you're hearing people denote 'strengthen the rotator cuff muscles' are they talking about neurological strength, tissue capacity, endurance etc?
Work on building capacity in different ranges of motion -- focus less on specific muscles.