Can companions use stimpacks if you give them some? I haven't been able to confirm this.

Yes, they'll use them.

Their limbs seems to get crippled same as you so they'll use them then. I've also put aid items like purified water and cooked meat in their inventories and they've used it.

Thanks for clearing that up for me.

They also use grenades and stealthboys, but usually not when you want them to, especially the stealth boy. Also, can anyone confirm it they use radaway and/or radix? I also keep my companion stocked with stimpaks/food/water, and have seen them consumed. Have also seen Piper smoke cigarettes (though she may do that on her own without me giving her any, can't actually confirm that).

The cigarette smoking is an idle animation. They'll do things like that if you hang around in one spot too long.