Well yes you can turn steppers while power is off but it's not very smooth with potential to damage the drives so not recommended really. One option is to use Pulse generator for manual operation of the motors but this gets involved but is do-able.

Yes you want Bipolar motors and how you wire them will determine how they perform. For best Torque you'd wire them in series but the speed will be lower for same volts and Torque drops off fast as speed rises.
Wiring in parallel is more common because it gives torque higher up the speed curve.

Fortunatly if you buy 8 wire motors you have the option to wire either way so if you wire series and find it's not giving what you require you can just rewire and run in parallel.
Depending on the volts you run then series will most likel suit a slow mill best.