rx580s don't require both power slots only the eight pin one. The hx1200i will be enough to power 6 cards. I have a hx 1050 and it's doing fine with 6 rx 580s. Plug them in one at a time checking the device manager to make sure the computer sees the card then power off and add another. Rinse and repeat.  Good luck.  If you run into problems , check and make sure you pci-e riser is good.  I have had to deal with a few bad ones on several builds. Also do not plug more than two risers per sata cable. The hx1200i should have at least 4 sata cables, use them. What kind of motherboard are you using?  Asrock h81 btc pro is the best board and is pretty much plug and play.