Late to chime in, I know...
The short answer is that on our boats you're fine to pull the middle slugs from your main's foot. Leave the main slug or two at the clew, and it would be best if these are metal or at least metal bailed. You do not need slugs anywhere else on the foot.
As for the shape - it depends on how your sail is cut. Racing sails generally have a boltrope on the foot, then a "pocket" in the foot to allow draft adjustment, but without loosing the wind out the foot. Cruising style boats with slugs along the foot don't usually have this "pocket" and the shape of the sail is often flatter, if not entire flat right at the foot, whereas there is definite draft cut in to the sail several feet above the foot. That said, you definitely can get better shape with a free foot.
Foot slugs are mostly leftover from days when the foot was laced to the boom along its entire length, carrying the weight of heavy wooden booms. Also, they do help to keep the main in place when flaked if the outhaul is quite slack...
FWIW, my main is cut for slugs - I took them out - it's better than it was.
~Aaron