2009 Mustang GT Pioneer Head Unit Guidance Needed

Pioneer Head Unit Guidance Needed 2009 GT

Hey guys,
I can't seem to find a thread addressing this specific circumstance but if there is already I apologize.
After thinking about it for months & doing a lot of research I decided to purchased the Pioneer AVIC-8000NEX from Amazon. I've installed aftermarket decks in all my previous cars but have never installed a HU like this before & have a few questions. My car currently has the Shaker 500 system. Ideally I would like to keep the factory 8" door subs if possible, but I have 2 12's and a 1200w amp I would like to run as well. Eventually I will replace the 5x7's in the doors and rear as well.

1) Is it even possible to run the factory door 8's as well as 2 12's in the trunk? If so, how? What would I do with the factory amps? I have the 12V to 5V step down thing but not sure where it gets connected.

2) On the AVIC-8000NEX in order to enable full functionality while driving, I know you have to permanently ground the parking brake. I think you just have to connect the parking brake wire to the ground wire, (both are on the back of the HU), is this correct?

Thanks in advance!