The motor shield is best suited for motors or geared motors that are less than 2A stall each.
It can run 2 motors at one time or one motor bridged for a total of 4A at stall.
I would not use the motor shield for servo drive only.
I would also not modify the ones that you have, you can use them for all sorts of cool things like pan and tilt for Sharp IR or Ultrasonic sensors or a gripper... etc.
Instead, get the Parallax continuous rotation servo. This is a servo made for robotic mobility using servos.
Radio Shack sells them:http://www.radioshack.com/product/index.jsp?productId=12798725#
or you can get them at Parallax website.