I'm building a PC for a friend using a Gigabyte GV-N960WF2OC-4GD graphics card which requires one 6-pin & one 8-pin power connector. Now I'm looking at a Cooler Master B700 ver.2 700W power supply which has two 6 + 2 connectors. My gut (and maths) say that this PSU will be fine but since I'm not playing with my own money I'm hoping for someone to confirm this for me.
Yes you can mix them anyway you prefer to.
Even if it has only 6 pins you can always buy a converter like this
For an in depth description You can read this article:
Some video cards have 6 Pin PCI Express power connectors and others have 8 Pin PCI Express power connectors. Many power supplies come with a 6+2 PCI Express power cable which is compatible with both kinds of video cards. The 6+2 PCI Express power cable is made up of two pieces: a 6 pin piece, and a 2 pin piece. If you put the two pieces together then you have a full 8 pin PCI Express power cable. But if you split the connector into two parts then you can plug the 6 pin part into the older 6 pin PCI Express connector and leave the 2 pin part unplugged. That way, your power supply only needs to have one 6+2 cable to be compatible with both 6 pin and 8 pin PCI Express connectors.