ok so im getting a car that has a 8 speakers

I'd get a set of comps for the front. Insert in oem locations for both mid/tweeter(Not sure if you meant a sep tweeter and a reg 2 way speaker in the door). Pick whatever rear 8's you want for the rear. For the f+r i'd just get a 4ch. Size of said amp depends on what kind of 8's you decide to go with. Then just get a mono amp for the sub. Depending on what sub you go with you could possibly just get a bigger 5ch.

You could always make 6.5 coaxils fit in the rear aswell.

you don't need to go active, you can run the front tweet/woofer with a passive crossover.

no coax in the rear, 8's in the rear are fine.

My car had a similar setup with tweets in the dash, door speakers, and subs in the rear deck. I have everything running off a single 4ch amp now with passives on the front and the rear speakers replaced with IB subwoofers running off bridged channels. If I wanted a dedicated big sub in the trunk, I'd still use the 4ch for the interior speakers and use a 2ch/mono for only sub-use.