The advantage of a slinger over an archer during the classical age was in his accuracy, precisely for the reason you gave: that it was usually fired directly (in a straight line) rather than indirectly (in an arch). An arrow could be heavily influenced by the wind or rain, whereas the sling was practically immune from conditions of use.
But as to your suggestion, there was no distinction amongst a unit of slingers besides experience. Yes, different types of slingers were better than others (i.e. Balaeric), but within that group (i.e. Balaeric) there was no distinction between elite and regular that I think you are trying to insinuate at. Even if there was, it is entirely a novel aspect of history that we managed to ascertain it and nothing else, and certainly not enough to warrant an "elite" slinger unit, such as Elite Balearics or Elite Celtic slingers. Slingers were considered elite by their cultural reknown and traditions, such as Balearic and Rhodisian, and because, face it, such peoples style of fighting was geared to that style. You wouldn't expect a Gaul to be as proficient as slinging.
It was the custom of the day, that when a slinger achieved distinction, he was promoted to the weilding of a spear or sword. These ranks, of true soldiers, was the area of distinction of the times.
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