How do you equip specific arrows types?

Quick use slots, or just Right- click -> Use from inventory ;)

I believe that once the next patch rolls out, you'll be able to use scrolls and arrows from inside backpacks, so you can fill a backpack with arrows/scrolls and place it on your hotbar. This may even work currently... I'll test it in a bit if I'm able to verify (and hopefully the latest patch will hit GOG today.)

Thanks.... :)