1. Melee units are any units with the attack type normal. This doesn't depend on range afaik.
2. Effective attack range is also affected by collision range, to prevent the possibility of a unit not being able to approach another unit to attack it. Furthermore, the reason that melee units have 100 attack range is that with lower ranges they have trouble with hitting moving(escaping) units.
Effective attack range:
attacker collision/2 + defender collision + attacker range
But never more than acquisition range.