I'm the same way AND I have a bad shoulder too. When I first started playing, I spent quite a while looking in the mirror and playing, even taking a video of myself, making sure that it felt and looked comfortable and natural. Believe it or not, if you're doing something to strain yourself, if you play while looking in the mirror, it'll be a lot more obvious...whereas if you can't see what you look like while you're playing, you might not notice right away. I don't really get the whole forearm-resting thing because on both my basses, it just doesn't seem necessary to me since the bodies are smaller, the size of a guitar, really. I've SLIGHTLY changed the angle of my right arm over the past few months as I've begun learning to slap (I hold my arm a tiny bit more vertical than diagonal), but if my forearm is too vertical, I start to hunch my shoulder. Not only that, but then I DROP my left shoulder slightly to compensate, and then not only does it look stupid, but it feels uncomfortable! :lol:
Try a few different positions, because you never know what you'll settle into. The way I'm built, "one size fits all" doesn't tend to fit me. I am almost 5'6 and about 95 pounds, but somehow despite not being that tall, I have kinda long arms for my size and big hands. I tend to forget that I'm considerably smaller than most adults. I spent an entire snowboarding season on a board that was way too long for me because I bought it based on my height and not my weight, for example.
Heck, I was ready to give up on learning to play drums because my teacher insisted on my playing open-handed (I'm right-handed, so, right arm hits the snare, left arm hits the hi-hat) and my left shoulder is still weaker than my right and kinda fragile, so I never felt or sounded as good as I wanted to even after really working on it. Finally I said screw it and switched back to crossing my arms over each other, which made it easier to hit the hi-hat consistently with my left arm. I haven't played in a while, but when I do, I feel more like myself and not like I'm fighting the instrument.
The "rules" don't always apply. I'll never forget the first time I tried my snowboard set up in the "goofy" stance, with my right leg forward and my left leg behind me, which is supposed to be for left-handed people (at least that's usually the case). Since my left LEG is also weaker than my right (yeah, it kinda sucks

), I would have trouble turning when I first started out. So when my feet were switched to the goofy/opposite-footed stance and I was able to lean on my stronger leg to turn, I was amazed at how much better it felt. I haven't gone boarding in years but I will probably leave my bindings this way.
There is nothing worse than not being able to accomplish a physical task because of various things which might limit you- size, injuries, etc. And there is nothing better than figuring out a way that works for you.
Oh yeah, one more thing. I carry all my tension in my shoulders and neck, especially my trapezius muscles. Stretch them out before you even pick up your bass to play...it will help loosen up your wrists. Also, sometimes when my left hand starts to cramp, I rotate my thumb so that it's slightly more parallel to the neck or I lift it off the neck completely just to take a rest. It is possible to subtly alter your posture without looking like you have a twitch!