Well, about ammo, I'm gonna give it a thought, but I have other things on my mind now.
In melee weapons it behaves exactly the same. If you want them to be more/less wounding you need to give them the tag. This is exactly why these ..._UNARMED items are there. Because in my personal changes to the game mechanics I have these "items" that allow soldiers and aliens to fight unarmed
These unarmed attacks actully deal very minor damage, that is why I don't see the reason to release them as a mod. They are just for flavor, you can't actually beat anyone up except a civilian or a sectoid. And I also gave them that tag and specified that unarmed attacks don't cause wounds at all (they inflict minor damage, but even when you manage to deal more than 5 damage per hit, there still will be no wounds).
Would you recommend to give your tag to a weapon with low damage?
You mean, so that less damaging weapons would still inflict significant amount of wounds? Personally I see no reason, because I see it as this: if the weapon deals little damage, than it's less likely to inflict more wounds. As I said above, I even prevented (already low-damaging) unarmed attacks from inflicting wounds even when the amount of damage would allow it. But in your mod it's your choice.