Just make sure you flash the bios to the newest version. Use the image from official MSI site. There is plenty of guides over the net on how to flash the bios in Z170A. The only other important change in the settings is “Above 4G Decoding” in Settings > Advanced > PCI subsystem. In the newest MCI Z170A Carbon this settings got renamed to something like “4G Decoding / Crypto Currency Mining”.
As far as I know getting multiple RX 4xx/5xx cards working with this board under Windows requires additional weird mangling of the AMD drivers (you need to combine different versions of drivers). I mine under Linux, and all the cards were visible without any problems.