The rear speakers in a car are really for nothing more than fill. you will get all your staging from the front, with good tweeters and all your depth from a sub. JL audio, as someone else mention, makes a stealth box for the coupes. It is very pricey, but makes awesome bass with minimal impact to space. Mustang - JL Audio
If you plan to stay with the factory amps, make sure you choose an efficient speaker. Choosing one rated for too much power will not perform as well with the factory amps. That is why i suggest the C2's. They work well with the factory power, and in a fully built system are good as rear fill speakers. So you can start with them up front, and if you expand they will work great in the rear.
5x7 and 6x8 are interchangeable. The shaker 500 uses the same size front and rear although the stock speakers are a little different. The rears have larger drivers as they run full range, the front have a low pass cross from the factory headunit. Unless you plan to use after market amps, replacing the rears is more of a pain and provides less "bang for the buck" .... just my opinion.