u can see on this wikipedia link:
https://en.wikipedia.org/wiki/Battle_of ... r_(311_BC)
they were that important on ancient warfare that they are mentioned appart from the infantry
and here another important mention on history:
Diodorus of Sicily writes:
“Their equipment for fighting consists of three slings, and of these they keep one around the head, another around the belly, and the third in the hands. In the business of war they hurl much larger stones than do any other slingers, and with such force that the missile seems to have been shot, as it were, from a catapult; consequently, in their assaults upon walled cities, they strike the defenders on the battlements and disable them, and in pitched battles they crush both shields and helmets and every kind of protective armour. And they are so accurate in their aim that in the majority of cases they never miss the target before them. The reason for this is the continual proctice which they get from childhood, in that their mothers compel them, while still young boys, to use the sling continually; for there is set up before them as a target a piece of bread fastened to a stake, and the novice is not permitted to eat until he has hit the bread, whereupon he takes it from his mother with her permission and devours it.”
with the balearic slinger, the cretan archer, the parthian cavalry archer, nubian mounted skirmicher, etc. There are a lot. I am not sure if all of them were mercenaries 
