I'm about 100% sure that ammo is infinite for settlers (at least they have been for me), so I usually outfit my settlers with the best weapons i can
but here are their uniforms (mainly so they can be easily identifiable for job assignments)
Guards: mining helmets, army helmets, or hard hats; welding goggles; bandanna; best armor surplus (metal->leather); and road leathers
Farmers: flannel shirt and jeans with leather armor, and a small hat (usually +1 luck) like visor or fedora, etc.
Scavengers: Gas mask and goggles with raider leathers and all raider armor
Provisioners: usually a spike or caged armor suit with an assault gas mask; i usually put my best surplus long range weapons on these guys; like to give laser muskets
Shopkeepers: try to outfit these people with casual outfits depending, usually somewhat related to what they're selling
i still got a long way to go, but this is what i like so far