Well it highly depends on what detailgrade you are aiming for and how good you are with
GS. Theoretically you can do everything with
GS with the right amount of skill and time.
I would use plasticard for the trim on the shoulders as it is much easier than creating it with
GS. Just use an existing shoulder and paint the contours onto your plasticard to have the right curve.
I don't know how detailed you want the insignia to be but it is probably the best to completely resculpt it in comparison to change the existing one.
Another approach to your problem might be a pressmold. Create a pressmold of the icon you want to replicate. Then use
GS to press the icon directly onto a blank shoulder using your mold.
You can even create the mold itself from
GS but you will need something like oil or other release agent to better remove the
GS copy from the mold. Or use Instant Mold or Blue Stuff for the mold for easier removal.
Once you have the copy, cut the excess away and glue it to the blank shoulder. No need for sculpting every icon by hand and with a good mold you'll have a hard time seeing the difference once painted.
But if you still want to sculpt them all by hand maybe browsing through some of those tutorials might help you:
http://fromthewarp.blogspot.de/p/archives-and-tutorials.html