I am clueless about car audio systems... PLEASE HELP!?

My friend wants to buy a car audio system and the model she has narrowed it down to is the Sony Model # CDXGT630UI. She would also like to "keep it in the family" so she wants speakers that are Sony as well but when we went to the store there were some Pioneer and Alpine speakers that I thought sounded nice. So my questions are:
1. How good are Sony speakers?
2. Does it matter if we mix and match? Will it effect the quality of either the speaker sound or the stereos capabilities?
3. BestBuy said that our dash thingy would need to be special ordered because we have a 2008 Chevy Malibu and it is to new so they dont have one. Has anyone ever seen a 2008 Malibu with a system? I dont want it to look ugly.

PLEASE HELP!!!!
So a couple of you have said Sony sux for speakers and what not soooo....an added question: What do you think about the Sony in dash stereo she picked? Any other brands better? She doesnt want hard bump just a nice loud clear sound.

Mix-matching brands will not hurt anything. If the Alpines or Pioneers sounded better than the Sonys, go for it. For what it is worth, Sony does not make a good car audio speaker. Alpine and Pioneer are better in my opinion.

Just be sure the OHMs are matched. If the Sony radio is putting out 8 OHMs, make sure the speakers are 8 OHM rated as well.