Do I have to give them ammo as well as weapons to defend themselves?

Does it help to give them armour?

just like companions the weapon they start with when you hire them as guards doesnt need to be stocked with ammo.. but if you give them a weapon of your choice they do need ammo.

Yes they need ammo to equipped and use the gun, but I'm pretty sure they run on old non-companion NPC rules. Meaning, they can use the guns with a small amount of ammo without ever running out. So if you put a weapon and 1 round into their inventory, the should be able to use the gun forever without running dry.

Thanks guys.

They don't run out of ammo. They need the right ammo, but it doesn't go down. I checked it out on all of mine today and the ammo stays at the number I give after a gun fight. I tried several different types of weapons as well including laser.

Great - good to know for sure