upgrading system?

Based off another question i had asked i've determined that i'm going to have 2 15s in my trunk/back seat area, the next thing i'm wondering is if a pair of 8s to replace the factory speakers would be a good idea or will the 15s and 8s cause cancelation problems with each other?  i'm planning to run sealed boxes for the 15s and probably the same for the 8s, i was also going to move the 8s so that they are mounted to my rear deck above the seats in the back window of the car by cutting a whole glassing a set of speaker rings to the deck itself and building boxes on the lower side so they look pushed thru from the bottom,  i'm trying to keep the subs facing the trunk, but i may face them forwards behind the back seat as my seat drops down, i'm also planning to upgrade the front speakers with a component system, and possibly another set of 8s in the doors, now is all this just really far overkill or would the 8s make a noticable difference? also i'm going to run adire audio 15s, with their 8s be a good choice as well? 

J~

so could i run a 10" with the 15s and not have cancelation issues? i dont mind tweaking things, i planned on an eq and a couple crossovers for the system already, i also do fiberglass work and am quickly picking up a vast knowledge of modifications to electrical and mechanical parts of my car, also i have around a 100 watt alternator in my car and a red top battery, i'm planning to upgrade to a yellow top under the hood, and possibly a second one in the trunk for the audio system, now if i buy 2 yellow tops i dont need an isolator correct? i'm also planning to upgrade the charging wire to 0ga and the main ground to 0ga as well as a 0ga wire between batteries fused of course, am i forgetting anything with that setup? i'm hoping to run 1 to 2 amps for the subs in the trunk and a 4 or 5 channel amp for the in car speakers, i'm also planning to run 4 ga power wires, now this should allow me alot of flexibility in my amp choices shouldnt it? my other question is if i have a second battery in the trunk i'm going to ground to the chassis of the car/body of the car, the audio system will also be grounded there do i need to keep all my grounds centralized i.e ground the battery and all amps to the same spot or can i ground the battery on the left and all the others on the right  and not have issues with sound quality?

J~

man your just full of good info aren't ya lol, wanna recommend some good 8s for the doors, and component systems? as well as a good brand for crossovers and possibly an eq?  i like that setup much better i had always heard that mixing 10s 12s and 15s will cause cancellation because they run in such close frequencies to each other, i've also heard let the bass come from the back and the highs from the front, now if i flip my subs to the front and build an enclosure, what do i need to do to make the bass sound clean and tight? i've heard that you have to foam/block off every hole and opening between those 2 areas so whats the real story behind this?

J~