volvo 850 system

Hi, i have a volvo 850 and have some audio equipment lying around that i would like to install.
I have completed afew installs in my time but this one will be a bit more advanced than my previous attempts so i thought i would post on here in an effort to get it right the first time.
head-unit old pioneer deh-545r

amps, pioneer gm-222/20a fuse sony xm-222/15a fusion ca-am20350/25a these are all 2ch amps i think the fusion is the most powerful and it is the only one with rca in and out.

speakers, jaycar 10" carbon fibre sub 4ohm 125wrms pioneer ts-e1690 6.5" 60wrms
pioneer ts-c1302 5.25" splits 50wrms pioneer ts-g1044r 4" 30wrms.

the head unit only has one set of rca's so i was planning on running these to the fusion amp which will run the sub then using the rca out from the fusion to the pioneer amp to run the 6.5". then i will use a line level to rca converter on the rear channels from the head unit to the sony amp to run the 5.25" splits and then the front channels from the head unit running the 4".
Running three amps off the head unit remote wire is not the way to go and a relay will be needed.
Wiring, i plan on running 4 gauge from the battery 60amp fuse to a distribution block as close to the amps as possible then 8 gauge to the pioneer and sony amps and twin 8 gauge to the fusion all will be earthed with 4 gauge for each, maybe a little overkill but ive got the cable so why not.
Will adjusting gains and filters on the fusion affect what is sent out from this amp to the pioneer or is it a pass through?
The fusion claims 175wrms bridged so this should be plenty for the sub however will i require a capacitor?
Also any advice on speaker power and earth cable gauges would be great. Thoughts on running twin 8 gauge to the fusion im only considering this as my distribution block is 1 4 gauge in and 4 8 gauge out. Just a budget system as i have all this stuff just lying around.
I would like to make the most of it however so if there is anything im not considering that would improve this system please let me know,
thanks twitchy.

Decided to buy a new head unit for this system want one with 3 RCA pre outs don't know if I should get a music only head unit or a double din DVD touch screen for music only I'm leaning towards the pioneer dexh9550bt if I go for a double din DVD i would like one that can play a movie in the back for the kids while I can still listen to music. From what I've seen only the cheaper ones play most formats like mkv mp4 etc and I've had bad experiences with non brand name head units in the past most of my music will be played from my iPhone 5 but I would like to know more about diga DVD music as it sounds like a good option for lossless music files do all DVD units play these? Any recommendations would be appreciated.