First of all let's revisit the Reeves drive itself and the moulder feed drive train. You say it turns too fast for the moulder feed. That tells me that the final gear ratio is incorrect for the use. You really want the drive motor to run at or near it's rated RPM, and then adjust the final ratio using gears, belts or chains depending on the application.
About VFDs, a standard V/hz VFD (what you have there) causes a torque decrease as the frequency is reduced and eventually you will run out of torque if you slow the motor down enough.
There is some danger of the motor overheating as the Torque Comp (Pn 13) is increased.