I think everyone can shoot at a large target.
You'd think so, but Large Targets aren't mentioned in the exceptions brought up to when more than 1 rank can shoot, nor is it mentioned in the rules for Large Target itself.
It might be an oversight, but we're 2 FAQs in and it hasn't even been mentioned, so for now, only 1 rank can shoot against LT's, unless you're on a hill, have volley fire, or any other rule that allows more than 1 rank to shoot.
Skirmishers however can probably all shoot at a LT since they only require LoS (and the LT-rules specifically say that everyone can draw LoS to them) and nothing about only 1 rank shooting (because Skirmishers don't have ranks).