I don't think that you can use IGPs of LGA 1155 processors when there is a discrete PCIe graphics card (or more than one) in the system. However, I'm not absolutely sure of that. You could try testing it.
Before you order the extra 7th monitor simply try plugging one of your monitors into the onboard video and see if it works, if not i wouldn't try to get an extra 7th monitor and just plug them all into your gpu's.
Under the nvidia control panel there should be some settings for to control the displays a bit, try messing around with that before buying the extra monitor to see if it will work.
It should be located in the Advanced tab of the BIOS under System Agent Configuration -> Graphics Configuration (I think that's right. I can't get the manual for the P8Z77-V Premium to download for some reason, but I looked at the Sabertooth Z77 manual and it should be basically the same).