Hellfire and other anti-tank missiles are not only used for armour
They have been used extensively and widely to hit even single targets, when the distance is great and gun is no longer the preferred weapons anti-tank missiles are used, also when they hit mud buildings they use ATGM
Indeed, as the Israelis have found, the shaped charge HEAT warhead of the Hellfire produces an extremely powerful yet highly localized explosion, and is very effective in taking out high value soft targets in densely populated urban areas with minimal risk of collateral damage.
When you are fighting in an urban environment, often the enemy will use cover, and sometimes the only thing that will reliably punch through the re-enforced concrete some sniper is hiding behind is a Hellfire or equivalent.
Getting back to the cal of gun attack helos carry, I took think 20mm is the bare minimum effective cal you can go down to. Any lower and you are giving up too much.
A higher cal gun not only gives you more firepower and increased flexibility in the kinds of munitions it can fire, but more critically, it also gives you a range advantage over the vast majority of infantry mobile anti-air guns, which can make a massive difference as being able to engage the enemy without them being able to effective fight back is no small advantage.
Looking at the Z10's gun, it looks to be 20mm at least, probably higher since that soldier looks to be cleaning the barrel of the gun, and unless he has freakishly small hands, the stick he is using looks to be easily 20-30+mm thick.