L-sit helps to reinforce rounded shoulders and good rows are impossible without ideal shoulder/back position position.

You can find the vidoes by the droves. People doing l-sit with there shoulders up and arms well ahead of the torso. This totally reinforces the high shoulder position (think of rounded shoulders as your back trying to slide over you scalp and taking the shoulders with it. The back and shoulder position are 100% related.) This is possible one of the worst positions to use as corrective exercises, as it does not only require superior thoracic stability, but is also a very difficult exercise.

As for pull-ups, its not so much that its working the lats, which are indeed internal rotators, but that they are extremely vital in positioning the scapula and humerus, are undoubtably a "back" muscle as it ties directly into the same fascia as the spinal erectors, and many other axial elements. When your shoulders are rounded the lats performing a complicated but unoptimal role in total body positioning, and need to reclaim their role as downward-rotators and depressors of the scapula... something that should be immaculately developed before plugging away at L-sit supports.

rather than making two posts, I will just continue this one for /u/Xereks

themoneybadger hit the most important element before giving what I think is some misleading advise in doing rows/pull-ups. Its all about how you operate throughout the day, and also importantly how you choose your corrective exercise. Until you can start transferring the benefits of your corrective exercise, and hopefully the hypertrophy and muscle activation it is helping you develop to your everyday life, you will just be recovering from your workout rather than recovering into what your total plan should encourage.

I definitely think you are on the right path thinking of the rotator cuff first. I would recommend "Secrets of the Shoulder" by Gray Cook to get specific and very contrary advise to the usual "do rows and l-sit bra" you get around here. By taking time to work on some corrective exercise and everyday posture at home and work, you can get your rotator cuff doing its job, disengage the upper traps/levator/others from helping the r.cuff with stabilization of the humerus, and start getting your back/ribs doing their job properly so the shoulders can come down the neck and compress with the back in a deep and complete retraction/depression.

If you don't wanna try "Secrets of the Shoulder," I'll just say you're missing out as it will work with dumbells, although you may want to buy kettlebell once you figure out a good training weight with the dumbells. You only need one weight and it doesn't need to be massively heavy, they just challenge the grip in a fantastic way, and grip and rotator cuff health go hand in hand.

This is getting long winded, so I will just say free weights are in general superior for this type of corrective exercise. I highly recommend Secrets of the Shoulder by Gray Cook to get you started and give some real guidance on how well your back is working before trying to correct the shoulders.