Some people told mixing the cards may work,for example if you have gtx 1060 6gb or gtx 1070 8gb.put to 1st slot any of them and try again, if it works then we are good to go,
No, it doesn't work.
Go from WIN10 to WIN7 - work for the moment, but there is a limit in just 5 cards in one rig.
I have 6-8 cards with win 7.