Which AMP do you have? A lot (mine included) have zone selectors.
Main Zone: Full 7.1
Other Zone: 2.1
Your sub should have 1 rca/phono cable.
Why have you got 6 speakers outside?
Assuming your speakers are 8ohms, and your amp does 4ohm/channel you could wire 2 speakers together... bin the other 2.
There's very little multichannel music around, so it's usually best to pump stereo out for music.
Unless you've got a projector in the garden and want DTS/AC3... in which case either make a switch box (
http://www.ebay.com.au/sch/i.html?_nkw=banana+plug+terminalwww.ebay.com.au/sch/i.html?_nkw=banana+plugs) or buy one.