The RM1000x comes with 4 PCIe cables with 2 6+2 connectors on each cable. How did you get 5 cables?

Anyways, using Sata/Molex for risers is fine on lower power consumption GPU's (all the ones you have listed come in this category). The burning problem that people talk of is the power adapters that come with risers - do not use a sata to molex adapter ever. Get a mix of Sata powered risers, and molex powered risers in your build.
The safest always are 6 pin PCIe powered risers, especially for GPUs that have higher power consumptions (200w+)