I have used a light dimmer circuit and also a series capacitor to reduce the voltage to shaded-pole fan motors to reduce their speed. (The dimmer circuit can genrate a buzz in the motor so for quiet speed reduction use a non-polarized capacitor in series with the line). The speed control is rather non-linear with voltage, but the motors operate fine under those conditions.
Shaded pole motors are induction motors and normally operate at near synchronous speed, but will continue to operate with a large slip at a lower speed if you reduce their voltage.