As for your power question, the best answer I have is "maybe". You should run a power wire directly from the battery to avoid noise.
If you're looking for an easy DIY install, the CS8A5 is NOT it. You will need to run all speaker wires from the back of the radio to the CS8A5 and back--not a real quick process.
As for the 8 speaker thing, when tweeters are connected in that manner, they are connected with a crossover, which eliminates any issues with the amp--when you connect the CS8A5 to the other speakers, you will connect the tweeters by default; so they will still work, and be powered by the CS8A5.
As a matter of system design, the CS8A5 is a great idea. It will add bass, and clean up your top end. From an installation point, you may want to have it professionally done.
Bazooka Mobile Audio