As far as the little cuff muscles, I am guessing "external rotation" since most people the interior rotation muscles, ie: drawing the humerus towards the body, is always stronger than sweeping the arm away from the torso outward. Lying L-flyes or shoulder horn work for a few sets/minutes at the end of say pressing W/O's (after these small muscles have be warmed up, with a #5 pound weight and some higher reps can strengthen them. It does not take revamping the W/O's really.
Can I ask why you were told about the weak rhomboids?
That said, anything from deads to bench shrugs to types of rowing work them.
IMO, if you work to strengthen the entire upper back, your rhomboids will be effected.