To answer your original question, “are upper body workouts helpful or harmful”, it’s not a “yes” or “no” question as it really depends on how you’re training. Importantly, what are your individual needs as both an athlete (general) and a pitcher (specific) and how best can we go about addressing those needs.
It’s difficult to offer specific advice without knowing more about you, how you move, how you throw, injury/medical history, etc., and for a guy like you, who’s already having some success and getting attention from scouts, I caution against taking general advice from anyone.
Having said that, there are some basic principles that baseball pitchers can stick to when training the upper body that you may find useful. Most (but not all) of the training programs I write for my guys abide by these principles. This is not an exhaustive list just the stuff off the top of my head. I’d probably need a whole article to really do it justice. (Hey, maybe I’ll do just that!)
(1) Don’t go super heavy on pressing movements. The heavier you go the greater the risk of injury. That doesn’t mean going heavy will guarantee you will get hurt. It’s just not worth the risk, in my opinion. Rarely do I program pressing movements under 5 reps and most will be between 6-10 reps.
(2) Make sure the scapula can move on the rib cage during most pressing movements. The scapula moves through (nearly) a full range of motion during pitching, so it make sense to train it this way in the gym. Push-ups, landmine presses and 1-arm dumbbell presses at different inclines are a great way to achieve this.
(3) Train the upper back at least as much as you train the chest. The upper back plays a huge role in decelerating the arm after ball release. Making sure it is strong by regularly including pull-ups, lat pulldowns, rows, face pulls, etc. can help with both velocity and injury prevention.
(4) The external rotators of the shoulder and lower traps need direct work. That means dumbbell/cable external rotations, and lower trap raises.
(5) The arms don’t need a ton of direct work. They will get plenty from pressing/pulling movements. I always LOL when I find out a pitcher has an entire day in his training program dedicated to arms.
(6) Most importantly: if it feels sketchy or you don’t like it – don’t do it!
One thing that often gets ignored is player preference. I know some guys that don’t like training their upper body much because they feel like it “tightens” them up (which they see as a negative) while other guys love it for the exact same reason! (In this case, they see it as a positive.)
For guys who see the “tightening up” as a negative thing, I’ll have them continue tossing a ball (just once or twice a week) during the off-season to maintain pitching-specific range of motion and a feel for their mechanics. I also use this strategy for anyone who is gaining a significant amount of weight (or engaging in strength training) for the first time.
For guys that see “tightening up” as a positive thing, I’ll usually shut them down completely from throwing for anywhere from 1-3 months (the exact time depends on a variety of factors) during the off-season.
To go back to your original question. You definitely can (and probably should) train your upper body. You just have to be smart about it.
As for gaining weight, I’ve been there, done that. Check out my response “How I Gained 60 Pounds and 7 MPH on My Fastball” in the “Weight Gains” thread.
I hope this helps!
(And if you want a bit more specific advice, feel free to shoot me a PM.)