Atl-alt is a little more complex than just a javellin. If you look at the back, there is a throwing board, that part you hang onto and use it to throw the spear. I've tried to make one before, but never could get it to actually throw, the nose always dipped way too much.

So to do properly you would have to add something to the guys hand all the time, and animate it properly. Doable, but a little more work than just switching models.
But as for slings, that IS something I've used fairly well. Never had much luck with a stave sling, just didn't get the hang of setting it.
A problem with stave slings is they have a set range, so if someone is charging you, you would have to keep resetting the range, but if there are a group massed somewhere and holding ground, they are great as you can zero in your range and rain hell down on them. I don't think stave slings would work very well in this game, unless you ignore how they really work and just treat them like an oversized sling with more power/speed.
For the simple sling, those things are great I love them. Wish I still lived where I did before, as its a 2 hour drive to a place I can actually use one, unless I want to invest in lead shot.

its all very light, soft sandstone here that just doesn't work with slings.
As for shot size, there are references (I forget which, never seen them, just read about them) showing Egyption IIRC soldiers throwing headsized rocks at short range. I use to be able to hit a mark at about 75 paces or so, they're very good weapons and almost better than bows in some points. Cheap ammo is a great point to them, and as long as you're fighting near a river, you have almost unlimited shot.
In a pinch they can be used as a hand to hand weapon. Put a large stone in it and use it almost like a club.
Shot, even smaller than an egg, thrown from a sling packs a LOT of force, and can easily dent 14gage cold rolled steel. Just for the fun of it I made a mockup of the side of a barrel helm (Not very scientific, but gave me a vague idea of the damage that can be done) put some weight inside it and tied it to stump. One shot from about 50 feet caved it in enough to
maybe kill a man, but atleast put him out of action for awhile.
Oh, and a side note to anyone that wants to try playing with a sling. be VERY careful, they are NOT toys. You are much more likely to kill yourself or a friend than you do with playing with firearms (Unless you are playing shooting at each other, but people tend to deserve what they get in those cases) I strongly suggest getting youself a good kettle hat style helmet made with moden metals and properly padded. (Think modern combat helmet with a wide brim) This will help to keep from braining yourself if you choose to try. But you're better off playing with a longbow.