Polk DB6501 Question

[Dubro]

My car is a 2010 Nismo. The stereo really sucks. So for the time being, until I save up enough for a good H.U. I'm going to install some Polk DB6501's which are really for the new H.U. that I get someday. (soon I hope) Hopefully they will help the stock H.U. sound a lot better.
Question is:
1. Should I install new wire to the stock H.U. from the tweeters and door speakers?
2. What size wire for the door speakers?
3. What size wire for the tweeters?
4. Where do I install the wire to on the stock H.U. I mean is it a harness with a plug end or do I splice into the wires behind the stock H.U.?
5. Is there a good known spot to put the cross over unit?
thanks for the help.