Yeah, ARC Trooper armour is almost a battlefield merit badge kind of thing. I believe it was a prototype for the Phase II armour, and during its early production run/testing they essentially just gave it out to clones who did well.

I don’t have a big problem with Cody deferring to Rex. Cody’s more of a manager where Rex is a team leader, so it’s natural for some delegation, including of tactical decisions. And they clearly have a close relationship so there’s a lot of implicit trust there.