I think the supply system is a good workable abstraction but I do think firing should be disallowed if your ships don't have the supply left for the shots.
As for movement in combat being "free", albeit limited to speed 1 when you are out of fuel, well, I think the fact movement during combat is on a much smaller scale than movement outside of combat makes it reasonable that there's no supply consumption. A small charge might be reasonable but if you really want to get realistic, ships out of fuel won't move slow - they'd just keep moving at the same velocity and on the same vector indefinitely!
Eesh! I don't want to think about the programming it would take to add a Newtonian movement system to the game, or the frustration of trying to plot intercept vectors and so forth for a fleet as you move.
