There are methods to obtain constant torque right down to 0 rpm for some VFD's, this usually involves some kind of feedback device in order to know exactly the motor rpm.
It has been possible to simulate servo type control in some applications such as elevator and crane hoists.
The HP calculation includes the rpm so it is also dependant on this.
I generally use 4 pole motors and take them up to the normal rpm of a 2 pole, i.e. around 3600rpm.
If you repeatidly use low rpm at high load for long periods, you may have to fit an auxiliary fan.