Is there a limit to how many units of T (bowmen etc) that can fire over each other?
You can get quite a few ranks in a still have the friends nearer than the enemy.
Also I wouldn't fancy standing in front of slingers that were shooting over head!

there is no a limit as it comes indirectly by the -2 malus.
With very few exceptions, indirect fire is performed between 5U and 15U. Within these 10cm (20 if with 28mm figures) you can hardly fit more than one friendly Unit in the middle. You can place 2 just to allow a Large Unit to stay.

The shooting of a sling is less curved than a bow, I agree, but not as direct like a crossbow or a harquebus. It is enough to allow slinger to support his line. This is differently achieved in DBx rules with the +1 in melee. Actually the supporting Psiloi of DBx are Skirmishers that throw from the rear.
At the end a Slinger Unit roll 1 die against Infantry and only if a Balearic or a good mercenary slinger (that is with VBU=3). So no very usefull in this role, probably better in the front.