6x9s in the rear's gonna require some decent customization. It's not impossible, but definitely call it work. If you're gonna do some customization, I'd recommend going with 6.5" speakers anyway (round speakers generally offer better sound quality and efficiency than ovals).
Unfortunately there really aren't many "direct fit" speakers for the rears out there any more... They're an off-sized speaker (5") that is more common in home audio applications than car audio.
4" speakers will fit with the screw holes lined up perfectly (or nearly), however they leave a gap around the top, so in order to maximize them you'd need to seal around them somehow.
5.25" speakers are another potential, however you'd need to grind a bit around the edges to fit them properly.
There are many good speakers out there... I'd advise each of you to go to a few electronics stores (along with a few dedicated audio stores) and listen to some speakers in the sizes of interest "blindly", meaning without looking at brand or price. Find what sounds good to you... If it's something out of your range, or that would require more than you can offer it (a decent amp, for example), move down the line until you find something that sounds good to you and suites your needs. Keep in mind, though, that the setup will sound different (maybe entirely) in your vehicle due to many factors... but if you get a good quality speaker, you can usually tune it to sound to your liking.
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