According to the wiki you can wear the following at the same time (as an example):
Head
2 x caps
1 x helm
6 x hood
Upper Body
1 x breastplate
3 x mail shirts
6 x cloaks
Upper Body (cheap)
6 x dress
3 x robe
3 x cloak
Lower Body
3 x long skirts
1 x greaves
Lower Body (no foreign items)
2 x trousers
1 x greaves
Lower Body (cheap)
2 x trousers
1 x leggings
Hands
1 x pairs of gauntlets
1 x pairs of mittens
Hands (cheap)
2 x pairs of gloves
1 x pairs of mittens
Feet
1 x pairs of chausses
1 x pairs of high boots
Feet (no foreign items)
1 x pairs of socks
1 x pairs of high boots
Feet (cheap)
1 x pairs of socks
1 x pairs of shoes
I tested it and it works pretty well as long as you tell the dorfs that the uniform replaces clothing. You can actually wear 6 hoods, 6 cloaks, 3 mailshirts etc. at the same time. If you think thatīs an exploit feel free to wear less. Personally I think that while itīs possible to wear, for example, 3 mailshirts, the weight should be a factor and maybe reduce movement or attack speed.
And itīs a good way to put all those goblin clothes to some use. At least cloaks and hoods...