I'd like to know the sizes of the speakers in the P5 -- there are tweeters by the mirrors, speakers in the front doors, and speakers somewhere in the back, correct?
The Harness, Kits and speaker size of all Proteges! will help you with choosing speakers and installation accessories. 5x7 is correct for the front and 6x8 aftermarket speakers will fit and 5.25 and 6.5 speakers will fit if you build yourself an adapter baffle. Most 3/4" tweeters will fit in the stock location. 1" tweeters might fit but I have never attempted it. Anything larger will not fit.
Please read the Rear Speaker Discussion for more information. In short, spending $XXX on a single set of speakers yields much better results than spending $XXX/2 on two seperate sets...when one set is mounted behind you and destroys the sound stage and imaging.