The generator must deliver the starting current with the minimum variation of the voltage and frequency, in all cases within the tolerance written in the Altivar catalog.
The power of the generators depends also on the acceleration ramp and the current limitation set in the ATV.
The current harmonics can overheat the generator windings because the RMS current is higher as compared to a sinusoidal current. Because of that, a line reactor is helpful. Ask also the manufacturer of the generator if there are integrated filters and to know the maximum harmonic distortion.
The bandwidth of the generator voltage regulation has to be adapted to not take into account the voltage drop due to the current harmonics.
Generally for the generator, 2 times the motor power is enough.
The generator power can be decreased (optimized) if the cycles of the application are well known and if it is sure it will not change in the future.