GPUs
You are mentioning Ethermine and Nanopool, so I guess you will mine Ethash (ETC or ETH) or may Equihash (Zcash). You are using 2 X RX 580 8GB if you decide to mine Ethash coins such ETH, ETC and so on, for a better result, you need to modify your bios cards (You can find a thread about bios modding in this forum, simply just use the search function).

OS
You should choose OS that you think more friendly to you.

Pooled or Solo
If you plan to mine major coin, do pooled mining, don't do a solo mining except you have to plan to mine newly coins.

Wallet Address
Would be better using your own wallet (safer). But if you plan to trade it in the market, exchanger wallet address is okay, it would reduce a withdrawal fee from your mining poll.

Mining Software
For client software, I suggest using Claymore.