Its all up to you and how much work you wanna put into this project. You could cut the forearms apart and add new shims/finishing strips.
The nice part about building a sandtrooper is that the small imperfections can be "camouflaged" with weathering.
There are though plenty of troopers (including myself) that have armor comprised of different armor sets. My TX suit has FX legs, arms, butt & cod with CAP-W chest, back and shoulders.