I think we could just allow X units per tree, X units per stone mine, X units per Metal mine, etc. They'll move on to the closest of the same object if the one they are tasked to is full. Or it can be a pathfinding thing where if units start bumping into each other too much they look for the nearest alternative source. As far as formations go, I agree completely with SMST. For instance, it would benefit your hero to be placed into a formation, because then he can't be singled out for focus fire--the entire formation is targeted instead of one unit