A soft-start module is a speed control that modifies the voltage waveform to the motor. Adding a speed control module in series with the soft-start would most likely destory the soft-start, or at least prevent the soft-start from working. The speed controller and the soft-start module both expect a sine wave (remember trig in school?) on their inputs. The output of either module is a sine wave with chunks notched out.
A soft start motor starts from the "off" to full RPM gradually? The alternative is the motor will try to achieve full rpm instantly. On a more powerful router, this could pull the router right out of your hands. Am I right in my assumption?
Are they also called "capacitive start" or is that another way of achieving the same thing?
I do have a "capacitive start" 1/3 hp motor on my "V-Drum" sander and it is not variable speed and it seems to be "soft start."
Note the fixed speed Hitachi M12CS(?) I think has soft start.