I just completed my 1st rig build a few days ago. I have 6 gtx 1070's, msi z270 mb, evga 1300 g2 power supply. It's been up and running for a few days now.
All 6 slots on my motherboard are already used so can I use a M.2 to PCIe adapter and add a seventh gtx 1070 to the rig? Are there any potential issues with doing this or is it fairly common?
Of course you can do that, it depends on mainly two factors.
1 being what sort of OS you are using? guess you might have already enabled all the required options on the mobo to run more than 4 cards, you may need to update the drivers based on your OS.
2 being getting a good M.2 PCIe adaptor, this is important because a friend of mine tried to build a same RIG using my config, but he hasn't been able to run the 2 M.2 slots on Z270P mobo!!
So make sure you get it on smoothly and like @philipma1957 said you may build 3-4 RIGs with same config