[Al Skierkiewicz]

Sanddrag,
So many questions, so little time. You can go to a junk yard and get a couple of wiper controls. They are big resistors (low ohms but high power dissipation) and they should be easy to wire up for what you need.
The router control device you saw is like a lamp dimmer for AC motors only. It contains a Triac that switches on for only a portion of the AC sine wave cycle.
Did you need to run all motors at the same low speed at the same time? If so you could buy a variable voltage supply. If the motors don't need direction control, a pot and a power transistor will likely do what you need. Make sure that the transistor is attached to a heatsink of some type. 2n3055 should do but anything Radio Shack has in NPN 5-10 amp range should work. Try a 10K pot to start and see what results you get. I think the junk yard is the best bet.