Whether you can use a soft-start would seem to depend on the type of motor. If the motor speed or torque cannot be controlled by voltage, I don't see how a simple resistor in the supply will work.
There are a variety of methods to control motor speed, including phase-shift controls for single-phase motors, PWM for DC motors, and VFD for three phase motors. The biggest problem in my experience with trying to control single-phase motors is the lack of torque at the slower speeds. Some have very low torque at start up, which is why there are different types of capacitor set-ups and centrifugal switches to try to counteract that. Electronic control of soft-start, such as one might use with a DC or three-phase motor, doesn't work too well with them.
I tend to associate effective soft-start with motor types that have high initial torque, namely DC, some universal, and multiphase motors. Unless your compressor is one of those types, my guess is that you will not have much luck trying to add soft-start. John