Bought a 2012 Chevy Cruze. Lookin to upgrade speakers, add a sub, and add two 6x9s in back. I want to keep the stereo the car has. Car already has 6.5" in each door, and two tweeters in front posts.
Can I add the 6x9s on top of what I already have, or will there be issues?

you can't add 6x9s because the stereo isn't designed to handle 2 ohm load most likely. this means you'll burn out the stereos internal amp.

you don't need more speakers than whats already in it. four 6.5s are fine.

upgrading the speakers without upgrading the head unit or at least using a 4 channel amp is a waste of time. without more power they won't be much better than whats already in it.

well I planned on an amp. Would that allow me to use the additional speakers or no?

6 speakers on a 4 channel amp is a waste of time as well. that means 4 would be louder than 2 of them making the 2 quieter ones almost useless.

run 4 6.5s because that is all you need. throwing speakers in the far back near the sub accomplishes nothing other than spending extra money.

so run the sub, and keep everything else on the stereo, or would an extra amp for the 6.5s help

it would help but unless you are running a good sub setup its not necessary.

how much you spending on sub setup?

haven't decided, but budget isn't an option. Because I know shit, I'll pay someone to install, but I wanna know I'm not getting screwed.

if you pay someone else for the install its going to cost at least $200-300 which is just a complete waste. it's much easier than you think it is.

also why aren't you wanting to replace the factory head unit?

I like the features already, and afraid of a terrible swap that looks bad. And I'd try it all myself in my 87, but with airbags in damn near every other of the car, and a broken arm on top of that, I don't feel comfortable tinkering with it much