The 20x rule is not absolute limit with VSD-E since it uses more advanced (recirculating) PWM output, which leads to less motor heating. Additionally in servo mode, the motor heating is really low. I'd say up to 25-35X could be used, or any lower ratio.
For maximum continuous torque, perfectly matching stepper has 7A current rating and as low as possible voltage. On other hand, up to 20A stepper could be used since drive can deliver 20A peak current in servo mode. Higher than 7A motor probably would be best for highest speed.