At stock a R9 280X pulls ~250 watts each and a GTX 1080 Ti is also pulling ~250 watts. That's 1500 watts of juice right there from your GPUs alone. You can undervolt, reduce the power limit % etc, but you still will need a beefy PSU to handle those 6 GPUS. You also need to make sure you have enough PCIE connectors since the GTX 1080 Ti uses 2 and at least some (maybe all) R9 280Xs use 2 connectors as well.
So then I should simply run 2 PSU? like 2 of the link I posted in OP?
I recommend a setup like this --- http://www.parallelminer.com/product/all-aluminium-6-1-gpu-open-air-mining-case-w-6-usb-risers-1-1200w-psu-kit-eth-zec/ --- with a smaller ATX style PSU to run the motherboard, CPU etc and 1-2 GPUs, with the other GPUs off the server PSU. Get one of these also to make it allot easier --- https://www.amazon.com/Add2PSU-Multiple-Power-Supply-Adapter/dp/B009P98Q8U --- makes both PSUs act as one when you turn your rig on.
