Hi guys I am building new, first rig.

 6 x sapphire RX580 nitro+ gb (over 0.6m USB 3.0 PCI-E 1x to16x Extender Riser Card Adapter with SATA Power Cable) and not sure how to plug them. I have win 10 now and one of the GPUs in the main slot in of the motherboard. I want to use my hx1200i to power it all but I am not sure if possible as I am not sure which cables/slots use properly

1. is 6 x 6pin slots and 6 x 8 pin slots enough to do that?
2. should I plug them 1 by 1 restarting my pc everytime or should it be okay to just plug the other 5 at the same time and windows will detect them?
3. if not then I have second PSU 650v and I am wondering if I can mix cables randomly, like plugging one 6 pin cable from PSU into one card and 8 pin cable from another PSU in the same card?

Thanks for the answer!