You can fit any sized speaker anywhere you want, it just depends on how much work you are prepaired to make it fit.
I'll not go into the whole "6x9's are shite" debate, as thats not what you asked.
Basically, to make them fit, you will need to cut some of the metal to allow them to fit. You will also need to make a templace (possibly from wood) to help them mount to the metal work, and to make it a snug fit. You might also need to make some adjustments to the rear panel cover, as there might not be enough room to fit 6x9's behind it.
If the speakers in a coupe rear quarter are the same as in a hatch, the standard size I believe is 8cm diameter, so to get 6x9's in will be quite a lot of work.