Welcome to the forum, Matt!
To address your questions:
1. Don't worry about the impedance difference. The usual recommendation is not to use the impedance switch on a receiver. Those just limit the max current, but as long as your receiver doesn't overheat and go into protection mode, you don't need to worry about it.
2. The subwoofer has various inputs to allow for flexibility of connections. It won't, however, power any of your other speakers. I'm happy to answer any questions that you have about those connections, but here's the short of it - connect the RCA subwoofer output jack on your receiver to the LINE IN on the subwoofer. The manual for the sub might say if you connect a mono input to one or the other of the left/right jacks. If it doesn't, you could use an inexpensive RCA y-splitter to connect the receiver's sub output to both the left/right inputs on the sub. Connect the speakers to the speaker outputs on the receiver.