Vex Replacement Gears

Are the replacement gears provided in the kit able to work on servomotors?

That depends on the specific gear in question. All the white gears are interchangeable between the Vex Motor and Vex Servo. The black gears are slightly different. The black gear with a mechanical stop (a square post on its side) is for the Servo. The black gear without the mechanical stop is for the Motor.