Yes I have fitted Rear speakers in my Lumina which would be the same as the SV6. The factory speaker size is 5" but I fited 6.5". All you have to do really is remove the factory plastic mounting bracket and replace with a 6mm mdf ring. There is a very basic wiring loom there for the original speakers so you will have to cut that off and crimp/solder on some speaker terminals (thats if you want to run the speakers off the H/U) Otherwise you can run new thicker wiring and attach it to an amp (which is what I did).
I don't have the centre speaker in mine as it is a Lumina but we have 2 SV6's at work for the reps and my dad has a SSV, I've driven them a few times and the centre speaker isn't that great. I would disconnect it if I had one.
Assuming you mean replacing the existing rear speakers, then as per Josh's post. If you mean adding speakers to the parcel shelf, not sure. That's where the factory subs are in my Clubby.
No distortion issues at all if you use one of the DDL settings.