Correct frequency determins speed, slow your generator down and all the motors slow down.
Probalby 50 to 100 RPM of slip is practical.
Look at some motor tags, bench grinder, table saw, and so on, the RPM will vary form 1775, 1750, 1725 and all these motors are threoredically going to spin 1800 RPM, no load.
Motor with lower speeds have more slip, and better starting torque, within thier speed group, unrelated to poles.
Motors and generators have certain RPMs 3600, two pole, 1800 four pole, 1200 six pole, 900 eight pole
I have seen 900 RPM 50 KW generators massive, compared to 1800 RPM sets.
Nominal 1800 RPM motors have four poles, nominal 3600 RPM motors have two poles, and half the starting torque, half the copper also.
All betts are off on hand tool motors, brushes and other things change the speed.
Also as far as slowing the generator down for different speeds the new computerised generators are a bit funky in this area.