10in subs will be tighter and punchier. I have a 12" Dual Voice coil sub. its tight and punchy but also has that looser response when called for. (ill just leave this comment here... )
I used to have 4" clarion splits in my falcon in the side vents that worked really well. Maybe you can fit 4" somewhere. There are low profile ones too.
6x9's were purely designed for space in car parcel shelves. You will never (or shouldnt) find a oval shaped speaker in ANY high end audio gear (non car). From an audio point of view they have more distortion and dont project like a speaker should. You are better off with a 6" round (2ways not splits) in the rear especially if you are going to run a sub anyway. Let your sub handle the bass and your rears (or fronts if you can have them) handle the other frequencies. If you cant have anything up front then id just run the rears and sub off the amp and leave it at that. Nothing wrong with sound coming from the rear only. not completely ideal and your buxom passengers in the back might not appreciate audio screaming at them. Also the trap door will project the speakers to the back of the seat so its not going to be as good as it could unless you drop the seats. Im sure whatever you do will still sound good and as its already noted our cars are V8's and make noise so worrying about the details i mentioned above is probably irrelevant as you wont notice "quality" loss anyway.
Another reason i ran the rears off the head unit is i couldnt justify another amp for speakers that point to the back of the seat, when the head unit is ample for filler speakers anyway.
Food for thought. Do whatever fits and works. Just get a good amp, good speakers, headunit and amp, suitable sub.
A good head unit and amp is generally key and can make avg speakers sound good.
hope that was confusing lol.