When hidden, stacked units are fired at, and there are multiple units with the lowest DR, is the one that's hit chosen randomly? I'd say yes, as the attacker does not aim at a specific unit, and because the defender cannot realistically which of the weakest units gets hit. But is this correct?
This is definitely not specified in the rules so for now it's up to the players. I would say random is a good way to go, personally. Defender's choice would be fine with me as well, as the defender could choose which unit was more "dug in" than the other one, and thus less vulnerable.