Cool! - I have a stocked rifle, my .40, that is very short, 12 7/8" to 13" pull, yet I have no difficulty shooting it, as well as those with stocks to 14". I do have to hold my shoulder proud with it or in a postion to fit any stock I'm using. Since the really short one is an English style stock, it tucks into the pocket, rather than sit on the shoulder itself. This is where hooks interfer with muscles.
I-too have shoulder problems and the flat butt plates fit best due this 'pocket holding'. The peck muscle is more a comfortable recoil shield, than the damaged shoulder joint itself.