Can a character (during a Rally Step) spend Fatigue to gain extra Maneuvers?

The Rally Step is a short pause in the action. Note that characters cannot even perform a single action during a Rally Step (unless the action has the Rally trait) and they only get to perform one of the items in the list, so they can perform one maneuver or perform an action with the Rally trait, or attempt a First Aid check, etc. Note that a First Aid check is a Skill check. A Skill check is a maneuver. You cannot make a First Aid check and perform a maneuver, since you can only perform one of the items in the list.