RE: Rookie questions
Each fire is treated separately. However, the advantage of firing all units in a stack together is the do it before any return fire is received and so no chance of a random disruption to the other units in the stack as an example. The negative is, if playing without hidden disruption, you can't move fire to other enemy units if a single shot disrupts the target and you decide to fire at other units. Personally, I always fire the stack unless I am going to target soft and hard targets with different firing units, as it speeds things up.