All I know is that 5-wire stepper motors aren't compatible with bipolar stepper drivers whereas a 6-wire motor can usually also be used as a 4-wire motor.
I have seen Threads here about using 5-wire steppers. I think they were generally low powered motors so I don't know if the drive mechanisms used would be suitable for your more powerful motors.
One issue to consider is that if you decide to replace the motors in future you will probably replace them with bipolar motors that will also need new driver boards and probably different software.
...R