for speaker wire i just duct tape 2 strands of 8 guage together //content.invisioncic.com/y282845/emoticons/fyi.gif.9f1f679348da7204ce960cfc74bca8e0.gif
If you have some extra 12 guage lying around, use it. If you're buying wire, get 16 guage and save the money. It won't make any difference. I have 12 guage running to my 6.5s because that's what I had on hand.
If you have to run that into the doors, you might end up hating life. Honestly, if you have it already, nothing wrong with using it, but lighter gauge is much more convenient.