First of all, i am sooo happy to see someone making boss/enemy -> player replacements that were not openly done.
second, the shoulders, what are you planning to do with them? it would not mess the animations up TOO much to kill what you made the top of DN's legs, then make it the natural shoulder making it slightly more like the proper build without any major problems.
okay the questions.
the more you move a part away from it's natural position, that would increase the area it moves about, so it would greatly look out of place, so you would have to make it static and attach it to the head/neck yourself.
you can in theory edit animations with a hex editor (some people are good at it) but for any animations that you try to change DN to Red13 would require it's skeleton and bone structure, or it would go awry, but might be able to change basic things over to red13 such as the attack to a bite, although I am not experienced in that field to say if that is possible or if it is how to go about it.
lastly, it looks like you might have a eye for some changes. Ever tried something like Gi Spectre as cid? that should be pretty simple since it follow's cid's skeleton easily, or maybe the iron giant as cloud, both have a sword and human build (although some parts may look weird since the lower leg is broken into two parts for the iron giant) or how about some changes that are not so 'easy' as it were? (hell rider VR2 would be pretty good since i didn't see any mods here that involved him, and he has complete parts, arms, legs, chest, leg, even a weapon!)
a change that would require you to be more, creative, as it were? such as making something like red13 with the ultimate weapon's parts since it in theory has the build, but not in that way, so it may be good to build up your experience in that field.
