I don't layer armor. But I do DFHack "tweak military-training". Skill > armor material in my experience.
Armour user skill doesn't increase protection, it just makes dwarves move better in their armour and get tired less easily while wearing it. That does mean they attack, dodge, and block more, but it doesn't increase their protection from a hit that actually connects.
From what I can tell, leather is always better than cloth for protection, but I'm not sure how it compares to adamantine, and Steel breastplates help more against Bolt/Arrow damage than adamantine. So for layering, using the maximums from the wiki, the best you can have it probably:
Armor
Steel breastplate - 1
Adamantine mail shirts - 3
Leather (adamantine?) cloaks - 6
Legs
Leather (adamantine?) trousers -2
Adamantine greaves - 1
Helm
Adamantine helm - 1
Leather (Adamantine?) hood - 6
Gloves
Adamantine pairs of gauntlets - 1
Leather (Adamantine?) pairs of mittens - 1
Boots
Leather (Adamantine?) pairs of socks - 1
Adamantine pairs of high boots - 1
If you don't have adamantine, then steel is 2nd best, followed by Iron/Bronze (Bronze is slightly stronger, but heavier and requires tin), then Copper, and Leather is last. Regular cloth provides VERY weak protection - it can be useful for keeping substances off of the skin, though, like syndrome-inducing blood, but the attacks cloth clothing deflects will typically be inconsequential anyway.
Of course you should always give the dwarves a shield, too, if they can use one.
Personally, I don't bother with all that, unless you modded in some ridiculous weapons or a race of siegeing giants or something like that, usually the following is more than enough:
1 Breastplate
1 Mail Shirt
1 Cloak
1 Helm
1 Hood
1 Gauntlets
1 Socks
1 High Boots
1 Greaves
1 Trousers
1 Shield
1 Weapon
The clothing items I make out of Leather. It provides complete coverage with at least chain armour, with redundancy in a lot of places, and provides as much plate coverage as possible.
I usually make all my armour out of the best non-adamantine metal I have, only making adamantine gear for veterans. I like to use my adamantine for other things, so the amount I put aside for military use is limited, and I want to save it for the soldiers who can put it to the best use.