Interesting. I'm not saying you are wrong, but in my coupe, the cutouts in the metal are a 6x8 opening, and all I have to do is pull the carpet from the side where it meets the door, and lift it up, and I can see the speaker. I don't have speakers in there, and i never did since my car is a base model, but mine are DEFINITLY a 6x8 opening. I would then asume that the limited either has different cutouts (unlikely) or some kind of adapter or enclosure housing a 6.5" speaker. I used to install custom sound systems for Sound Advice, and I have a complete system including rewiring the speakers, so I know a little bit about the Crossfire Audio system.
As for the fronts, 6.5 is the same as 6.75. The car audio industry reffers to these as an oversized 6.5. That means, any 6.5 will fit. I used MBQuarts in my car and they are a perfect fit (German car, german speakers).
I would suggest going to a more reputable shop than Best Buy or Circuit City. Maybe not more reputable, but a shop the regulary works on high-end cars like mercedes, ect. Believe me, this makes a difference.