On my map, one of my clone units has a shotgun and a blaster rifle. Unfortunately, these guns use the exact same model.

What is an ideal model to change the shotgun to (for both the Arial Trooper and the stock engineer) preferably only having a bit shorter barrel, and how do I do it?

i think fai22's carbine is a good model for shotgun, just my personal opinion

concerning the "how to":
a searchresult for weapon;model;change

I don't mean changing the actual weapon, just the 3d model used for it.

Open up the weapon odf and change all the model references to whatever other model you want.