The higher pole IM are less efficient, so you will find only 2 or 4 pole motors to be suitable for VFD. With 6 pole and above you can expect troubles if you wan't to access intelligent functions like sensorless vector control. Also the high pole motors are usually made for star/delta direct power, therefore they have larger air gap to increase the reactance of the motor, thus limiting the start current.
A performance IM motor for VFD has a tight air gap and small number of poles.
The above torque/speed characteristics is for Lenze 4 pole motor tailored for VFD use. It has a nominal frequency of 120Hz, thus 3600 RPM nominal speed, then you use a quality gearbox to reduce the speed.
But if the IM is driven by a VFD, pretty much any speed (or frequency)
can be generated and the torque and RPM can be tightly controlled.
Not really. Bellow 15Hz it will stall, also it is not advisable to use an IM below 25HZ without forced cooling. Above the rated frequency the torque begins to drop substantially. So, there is just a small window 30-60Hz for regular use of a 50Hz IM.