is it possible to reduce rpm of AC motor with the help of rheostat?
It's more complicated than that. Most motors don't power the mechanical device directly, the power goes through a gear reduction or uses belts etc to actually deliver their power. A good example is a washing machine, So by reducing the rpm of the motor it reduces it's power output. This is often not on a direct 1:1 scale. A 25% drop in speed might result in the motor not having enough power to continue to operate. Note that DC motors are different. Perhaps you could explain what your thinking of doing, and why. Good luck!
Generally no but depends on the type of motor. An induction motor speed can only be changed by changing the frequency of the supply. It is probably not ideal to try it with a universal motor because it is serial wound but under some circumstances, you may get some results.
A rheostat will only limit current which limits torque. To control speed requires an inverter. A rather expensive device compared to an SCR controlled DC motor but it is worth it in some applications
Here's a link for store bought converters:
Yes, You must use one that can handle the amp draw. The bigger the draw on the motor the bigger the rheostat must be.
