I'm looking into purchasing a component speaker system for my '12 GT with factory radio and speakers. I'm having trouble finding documentation on which speaker size I need to go with. Crutchfield says 5 1/4" for both the fronts and rears, but I've seen posts/articles that say 6x8s will work.

I want to stick with a component setup, and I understand the 6x8s are coaxial, but is there enough room to step up to 6 1/2"s?

Also, will I need mounting brackets for either the 5 1/4" or 6 1/2"?

 

Yes the 6.5's will work, I have 2 installed in the front doors, but you have to make your own adapter and modify the stock setup a little. Here is a picture of what I did to make my component set fit. I have a writeup on it somewhere, might be on another site though, Ill see if I can find it.

The black screws were the original holes, as you can see they don't fit the same as the original speaker holes. I used mdf for the bracket. pretty easy to do. I hope that helps.
a 5.25 will fit a lot easier, brackets are already made and many times you get them included when you get the speakers. Some can even fit a tweeter without modifications.