I don't think there will be much downside to using Windows 8.1 vs. Windows 10 provided it is setup correctly (the right drivers installed, the right software, etc.). Many people will suggest using Linux. If you are planning on mining altcoins, maybe look at EthOS. It's fairly straightforward to setup.

Bonus answer: You need to fill all of the pin slots on the GPU. I would recommend looking up a Youtube guide on how to connect GPUs to PSU correctly if you are unsure. It's difficult to explain it only with words =/