Should be no problem regarding the impedance - but why 5x7 in the back? The stock speakers are 6x9.
Also, if you want to put 6.5 component speakers in the front, you will likely need some sort of a mounting adapter because the stock speakers are 6x8 size (as far as I know).
Actually, this information is based on posts of other people on the forum, since I have not replaced any of my speakers yet. However, if someone can chime in with relevant info, I'd be very interested too, since speaker replacement will be the next thing I plan on doing as well.
There was a thread with some instructions by KKMmaniac about how to take off the door panels.
2004 Titanium gray 6MT
"A man is the less likely to become great the more he is dominated by reason: few can achieve greatness - and none in art - if they are not dominated by illusion." (Mr. Doctor)