Can units actually make aimed ranged attacks in Fortress mode? In Adventure mode, there doesn't appear to be any preference to the adventurer getting shot in the head, and the adventurer is unable to make aimed, ranged attacks.
I've had my Marksdwarves pelted by elite archers before, and they took wounds all over the place. They were behind fortifications and on towers so if it's true only elites can shoot through fortifications, they would have been shot by elites only.
Also, leather armor can't be stacked with breastplates due to them both being shaped armors. Chain shirts and breast plates are the best option then, as they're the only option for stacking. Well you could do leather armor with chain shirts on top, but both should be flexible types.
I don't know if Leather has a whole lot of benefit, or moreso than regular cloth, with the current material system.
Personally what I opted for was to only cover the vital organs of my dwarves with full armor (chain shirts, breast plates, metal helms), and have leather for the limbs. Since bolts and arrows shouldn't sever limbs (I say shouldn't; never say never!), organ and spinal damage should be the only major incurable injuries you have to worry about. I suppose they could still bleed out through the legs and arms (and the non-defensible throat), but protecting the heart and brain is most important.