I have room for a 7th GPU in my rig, however the only 8pin PSU slot open is an extra CPU connection. I have an extra 8pin to dual 6+2 that I can use.

Is it safe to run a 1070 Ti off of that?

I also have the option of using a 6pin Peripheral slot with a 6pin to dual 6+2pins, but I'd have to buy it.

I assume each rail is capable, but I just wanted to check to make sure.

I would have plenty of overhead on the PSU. Total running would be 1,000W.