On Slingers:
I did some research and came up with these results: The furthest range of lead slingshots (which were used by the funditores, for example) was 400m, it could hit a target most accurately at a distance of less than 100ms. Apart from experienced slingers like Rhodians or Cretans most preferred a wide fire of many slingshots on defenders on walls or close formations without too much aiming, though. Roman slingers also used two main types of slingshots, oval ones and spindle- like ones. Most slingshots weigh between 30g and 70g.
Persian slingers (which I also suggested as a unit and which might be similar to all Eastern slingers) still used stone slingshots by the time of Xenophon, while Rhodians already used lead. The Persian slingshots had a greater impact, but the Rhodians had twice their range, which was a big advantage obviously. Also, slingers should have a LOT of ammunition- we have proofs for both Rhodian and Roman slingers that they produced new slingshots DURING the battle. Not so useful for the mod, but also interesting is that letters for their home cities, names of the soldiers, ironical or even sexual jokes or emblems of the gods and godesses were inscripted on many slingshots (of course these would have been produced before the battle).
As for the classical hoplite phalanx: Generally it should be noted that men stood about 5-6 feet apart from each other (only 4 feet in the Macedonian phalanx) but that a phalanx and the hoplite shield were also useful in loose formations (as shown by Greek marines who used the same equipment despite not being able to fight in formations). A usual hoplite phalanx would fight between 16 and no less than 24 ranks deeps, because keeping the formation together and preventing the flight of those fighting in the front was the main target. The Spartans, however, were more confident and mostly fought with a formation only 8 ranks deep. To counter this, the Thebans deployed the Sacred Band at 50 (!) ranks deep at Leuktra.
As for underhand/overhand... I think in the hellenistic period overhand was favoured and RTR VII also used that if I'm not wrong. But actually both were possible:
(http://upload.wikimedia.org/wikipedia/commons/9/9a/Two_hoplites.jpg)