Different non-reference gtx 1070 cards

I'm building 4x 1070 mining rig and have few questions:

1) Can I mix different non-reference cards (like gtx 1070 Dual from Asus and 1070 from Gigabyte) in one rig?

2) If I want to add 2 more GPUs to make it 6x gtx 1070 in the future do I need to order one 1300W PSU or it's better for me to order two 750W PSUs?
3) Is it more efficient to make 4x 1070 rig or 2x 1070 + 1x1080 (not Ti version - too expensive right now for me)
4) I have Athlon II processor, what motherboard should I purchase to make it work with 4 to 6 GPUs?
5) Is it reasonable to buy SSD for this kind of rig? or usb is just enough? I've heard ssd saves power.
6) Finally. What kind of risers do I need to buy?

Edit: I have one GTX 1070 Dual Asus GPU right now, but I'm going to buy more of GPUs this week.