If a dwarf is looking for something to wear, whether because of wear to their clothing or because of a player-enforced uniform, they should assign themselves items with a higher armour level first, replacing items of a lower armour level. For instance, a dwarf has just been drafted into the military and been assigned the metal uniform. She is currently wearing a full set of ordinary clothing (level 0), because civilians will only wear items of armour level 0. Upon assigning herself the helm (level 2, for metal) and boots (level 2, for metal), she notes that they are of a higher armour level than the headgear and footwear she is currently wearing. So she takes them off to make room for the helm and boots.
With the current armour levels, this would make dwarves always go for a breastplate over a mail shirt, since it's higher level than a mail shirt and the metal uniform only calls for one item of upper body armour. In that case, the armour levels for those two items could be swapped, or made the same to replicate the current behaviour. The metal uniform could call for two upper body items, but that would make the uniform even heavier and result in two mail shirts being worn if there were no breastplate available to fill the first slot.