This is just an observation I had, but it seems that when in the battle ready animation (unsheathed weapons), the shoulder pieces generally will not rotate as much as the shoulders of your character. As a result, this does result in a lot of clipping with the back of the characters arms. It works with some models, such as Mighty Chudan's shoulder piece, but shoulders such as Troll King, Epaulets, Arm Cops, and Pauldrons do not rotate as if they were attached to the shoulders of the character.
E.X:
One thing noticeable with Chudan, is that while it rotates well with the shoulder, it does not in fact have a hinge interaction with the bicep of the character model, whereas Troll King and the Nord shoulder piece do.
So it seems that the animation team is using multiple joints for different pieces of shoulder armor. I think that's a bad idea, at least for the generic shoulder piece. If you take a look at the Engine guardian monster set shoulder you'll see that it has a generic shoulder piece, and a model piece that is attached directly to the bicep. But for the generic shoulder piece there should only be one joint, who rotates forward and backward based on the shoulder rotation, and hinges up or down from the shoulder point of contact based the the angle between the bicep and the Y-Axis.