For some reason, WoW Model Viewer keeps bugging with my Shoulders and it won't let me make one :(
Yea, it is from time to time. Sometimes it buggers the shoulders, sometimes it doesnt. if your really interested in doing it, I found the best workaround for thsoe issues is to load the shoulder models as their own, and have the background as a image of your toon shoulderless and change the angle and such till it matches. abit mroe work, but with races like goblins, the oversized shoulders just wont fly.
also some of the really animated shoulders (Like the Terrorblade T8 Rogue tentacle shoudlers) can bug if you have too many animated or flashy pieces of gear, cause model transparencies or blocking. I still have little idea how to fix that.
Final thought on the subject is if your having issues with model viewer, go find The warcraft Machinima tool, You can use it to hide WMO/Terrain (Buildings, trees and other infrastructure / Ground and land models) and set the skybox to capture something better.