I fitted 6.5 inch components right in. 6.5 inch speakers fits straight on both front and back. If you want 6x9s you have to modify your parcel shelf at the back, then there's the problem how the parcel tray will sit on top, remember you don't want nothing to be touching your speakers, and interfere with them... If you want, when buying take your original speaker and compare the hole position for you to screw down the speakers, if they match you'll be fine.
Yes, you do have to remove the rear seat to gain access to the rear speakers, not to hard, 2 bolts hold the bottom under the rear seat, then once that's off, exposes 2 other bolts on the bottom of the top piece, one on either side. Lift and bend seat a little then slowly force out. Remember to lift as there is a hook that holds the rear seat in the position it's in.
The parcel tray may be a bit of a bastard removing, push down on the sides and pop the pillar clip out then gently bend a little bit and then slowly force out again...