PCI-e Power - Does 6 + 2 Equal 8?

Short answer:
Yes

Long answer:
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:
http://www.playtool.com/pages/psuconnectors/connectors.html

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.