If you want to start fires, please connect all your GPU's with SATA.
If you dont want to start fires, please read this post: https://www.reddit.com/r/EtherMining/wiki/hardware/risers

Sata cables are made to power HDD and SSD. Maximum of 54 watts/4.5 amps.
Molex are capable of running 1-2 GPU (or 3...), its safer but not perfect.

Use the PCI-e cable for powering the riser. It's the best way.