As others have mentioned, using the 6ohm or more setting with sensible crossover settings for all speakers will not be an issue for the RZ50. It will run a bit warmer but not really hot. With your receiver connected to your network, you can use the web setup feature to check the temperature of the receiver at any time when using it.
Without any movies or music playing while using my Mac to web browse, my Onkyo is fine. It may go up 10-15 degrees Celsius if I get into movies or music at considerable volume levels. But, I do use the 4ohm setting as I'm using very modest speakers with sensible crossovers and am not trying to blow the roof off of the joint. When I switch to the 6ohm or more setting, things can warm up an additional 10-15 degrees Celsius and the fan will turn ON and OFF as needed but I've never had the Fan Speed go any higher than Low.