Some servos want that pulse to be reversed from the others, but they all want to center at 1.5ms. You have centered them on the 1.5ms timing pulse to be full stop. You have to issue pulses for at least 20ms at a time to keep the servos from getting confused. Then you can go off and do other stuff and come back to issue control pulses to the servos again. The foam type are lighter and thats why I used them.
All servos come with a variety of servo horns.
Pick the one that looks easiest to use and bolt one to each of the wheels in your favorite manner. Be careful to center them well. The back wheel is a single caster. I have this switch connected to the Stamp port 2, it is sampled after the IR sensor is and has priority over all other sensor inputs. Two light sensors are placed such that the robot can tell if its darker to the right or the left.