My understanding... why you polarize a system. To change the polar magnetism. It's some wizard stuff that I'm not clear on or agree. I've always done it with regulars anyway (fields etc). Because they said so.
Putting a 12v on a 6 volt system has complications not related to polarization. Going from positive to negative ground is less important. You can do that with 6 volts and don't need to polarize anything on a regular stock cutout.
On a diode.
https://learn.sparkfun.com/tutorials/polarity/all
Battery has poles already setup. Generator spins in one direction only no matter the current direction. Greater minds will add.