12 18 gauge wiring?

I have 4 speakers coming in soon for my car.  150 watts RMS. 

I was curious as to what guage wiring I need to run these.  I had some 12 gauge streetwire wire running to the fronts but needed more for the rear.  Then I remember being on the site months ago and reading somewhere that you never ever need 12 guage wire to run speakers.  Even at 150 watts RMS.  I have 2, 1000 watt RMS subwoofers as well so instead of buying more I switched out that 12 gauge wire from the fronts to the sub box to wire my subs. 

However now that I read the sheet they suggest 8 Gauge power wire in the diagram for the subs...is that not overkill.

Basically my questions are:

1)  What size speaker wire do I need for each of those speakers

2) Does it matter if I get lets say Monster cable instead of Streetwires

3) I have heard that all 4 sets of wires from the amp should be the same length.  Is this true?  That would create lots of wire mess in the trunk for the rears.

4) Is 12 Gauge fine for each 1000 watt RMS subwoofer?

Thanks in advance!

Thanks for those answers.

And I wasn't thinking.  I am running 2 ampilfiers at 1000watts each for my two subs.

The part about is Monster cable fine.  Sure for a cash perspective I understand but my friend can give me 50 feet of the monster cable for free so I might as well use right? 14 gauge