Swing/club head speed is/are the major determining factors when it comes to choosing a shaft. A player who swings 50-60MPH will not benefit from a stiff shaft, regardless of how tall or athletic they might otherwise be.
Best bet when a player is starting out is to pick up a standard shaft and try to develop good swing mechanics. Once they swing consistently (motion and speed), then I would start tinkering with different stiffness, kick points, etc.
One more point for new players - don't try to kill the ball and swing 1000 MPH. You are swinging a club that (including your arm) is about 6-9 feet long. The speed of the club head will be plenty fast without trying to crush every shot. Focus on making perfect contact, every time. Most experienced players will tell you that they hit further, more consistent shots when they relax and let the club do the work.