I have the same PSU and Mobo combo.. Had some issues with the EVGA not firing the BTC-81 if I connected the Molex to the mobo. I'd test this right away when you get your CPU and ram installed. I am RMA'ing the PSU so just a heads up it may just be a defective unit i've been dealing with.

As far as risers go, you only need 1X regular risers.. The Molex takes care of the rest unless you want to add 1-2 powered 1x risers just to be safe. PSU #1 should power the mobo, cpu, molex and up to 3 GPU's. GPU 4,5,6 should be powered by a second 1000-1200w PSU. This setup will give you both breathing room and the ability to expand/adjust if you are able to drop voltages on the cards.