I quite like the distinction made, and it makes sense. The problem is that this is a very simple game that works brilliantly, and there are a myriad of such small changes that could be made to the game. I personally am happy as the rules are.

I do agree. With the exception of rules concerning the phalanx, I think the game must remain as it is, simple and fast (I don't know if all this sounds very english, excuse me, poor frenchman that I am).

Slings and bows of antiquity were pretty similar...the world record distance for a projectile fired from a sling is over 1,400 feet, so reducing their range in the game doesn't really match with their use in history. They are a long range, volley weapon, like the bow, and with lead ammunition prevalent in greece and beyond, just as deadly.

Was there a significant difference in rate of fire between sling and bow? Was the sling a faster firing weapon? If so then should a sling unit be able to move and fire under these rules?

I'd be concerned that at the scale of difference proposed, slings and bows are not very different at all. In DBA for instance, LI, LS and LB are all just called "Psiloi", or light skirmishers. I think one if the things that's weird in CC:A is that there are so many different Light Units already, yet no real distinction between general heavy infantry, hoplites, phalanx, and legion.

In some games, Balearic Slingers hit in SWX if battling back. In our games, we let Numidian elite LC hit in SWX in close combat, but they get no ranged fire, to make them a bit more like MC, which is how they were used.

Actually the sling was an excellent weapon for it's time, in no way inferior to the bow. As far as I know, the firing rate is equal, or maybe 1,5/1 for the slings. The effective range distance is also in favour of the slings. However it is not possible to fire while moving, or that will be at the cost of drastically reduced effectiveness. Whoever is interested in history and usage in that simple but deadly weapon may find a lot information and illustrations at: http://slinging.org
There are also a lot of manuals for do-it-yourself slings if you are eager to try yourself. I braided myself one.
The problem with the sling is that you have to have a lot of practise to use it effectively (this is why Balearic slingers were such an asset to an ancient army). Another issue is that it is more of a scirmish weapon, than one that you can use for firing intense volleys as a group, thus gradually loosing popularity in the late years.