you can wear the leggings with the greaves, i think. again, try removing any cloth pants you may have equiped.
You cannot wear leggings with greaves since they are both shaped pieces of armor.
Only masterwork and artifact armors give a deflection bonus of 2x and 3x respectively.
Optimal armor set up:
You can wear 2 caps, helm and 6 hoods on your head
or a mask and helm and 6 hoods.
You can wear 3 mail shirts, 1 breastplate and 6 cloaks
or a ton of capes on your body.
You can wear 2 pair of trousers and 1 pair of greaves on your legs
or 3 long skirts instead of trousers.
You can wear 1 pair of mittens and 1 pair of gauntlets on your hands.
You can wear a pair of socks
or chausses and high boots on your feet.
Greaves > Leggings.
I like wearing 1 mask instead of two caps because a mask will cover your face and the helm can cover anything your caps would.
Cloaks are better than capes, but I have yet to find any cloaks in adventure mode.
Mittens > gloves.
High boots > low boots, chausses > socks
Be aware that you are probably not going to be able to wear 6 cloaks since you will most likely want to wear at least a backpack and/or quiver.
Hope this clears things up.
Also, I'm amazed that I could recall that from memory since I just passed out and woke up and I am kinda groggy.