16 Gauge for speakers and 8 for Amp is good enough since you won't be pushing alot of wattage. I do recommend howeverthat you don't hook both the amp and headunit to power the speakers at the same time(distortion and eventual speaker failure can happen). If you are going to use the ISO plug to pass the sounds from the amp to the speakers, that won't be a problem. Make sure you know which ones are + and -.
That sketch is what I was planning on doing to send you. Good luck and shoot some pics as you go along. It could benefit alot of people here.