I would not use a dimmer or a router speed control on any machine that has any computerized functions in it.
I wouldn't hesitate to use one on a vintage all-mechanical machine - one that has no electronics/computer in it. (Your 401 is one of those)
If you have a jackrabbit machine, a router speed control is a magic piece of equipment for helping you to learn FMQ. No matter how hard you press the pedal, the machine is only going to go as fast as your pre-set speed. Cheap and easy way to get the hang of coordinating all that activity. :)