How do Light Foot attack and defend in Chainmail?
Looking at the Combat Tables (p.40 of the 3rd edition) I would say:

A. They cannot attack. Light Foot already have only a 1/6 chance to kill any enemy troop type; anything worse would be below the resolution of the system. A generous referee might allow them to kill other Light Foot on a 6.

B. Add 1/6 to the "to be killed" chance. So Heavy Foot attacking Light Foot normally kill on a 5-6; attacking Fatigued Light Foot they'd kill them a 4-6.