I think you'll have problems fitting 6x9's in the door due to the depth of the magnet and clearing the windows. As well as all the modifying and cutting and stuff it would require!
In the rear, do you mean either side of the parcel shelf where the OE speakers would be? I beleive there isn't enough room there.
For the front get a decent pair of 13cm speakers which will replace the OE door speakers, for the person on a budget (like i always am) I can recommend Infinity Reference 5002i, though these have now been replaced witht he 5012i. If ya got the money and the will to fit the tweeters component speakers would be better.
As for the rears, people "in the know" say rear speakers add nothing to the sound in a car and can ruin the sound stage. The best thing to do is have good components up front and a decent sub for the bass. I've always got passengers so i just put the OE ones from the front in the back at low volume just to fill in.
The OE size in the rear is 10cm but you can get adapters to fit 13cm which are £10 from halfords. But these aren't the best solution as they do rattle a bit i find.
As for 6x9's, if you're not going to be fitting a sub and want more bass, the conventional thing to do would be to fit these in the parcel shelf. You'll need to either buy a pre-made shelf or stealth shelf from http://www.autoacoustics.com
or similar. Or you could get a sheet of suitable MDF, trace your shelf, cut and shape it, carpet it yourself etc etc. The standard shelf will not be strong enough to support speakers or provide good sound.
Hope that helps a lil.