Front Stage Infinity

OK so I think this will be the first time I have actually asked a question about my personal vehicle and would like some input. My current equipment is as follows

Alpine CDA9831, Infinity Kappa Pefect 6.1 in front doors, older kicker impulse 5x7 in rear doors, infinity 7521 amplifier going to front speakers (75x2), 2 Kicker 05 12"L7's on a kicker 1200.1 amp all tsunami professional nterconnects all kicker 0/1 and 4 gauge (two runs)

the question is this I want to change out the rear speakers to kappa 5x7's and move the 7521 amp to them then I was thinking about getting the 7541 (75x4) and putting that on the kappa perfects (front channel) and getting 2 8" infinity subwoofers in the kick panels (rear channel) I have two questions 1 do you guys see any problems in this setup i.e. using the 4 channel for that purpose and the second should I run the 8" woofers off of the non fading output so that they adjust with my subs or off of the front output so that i retain fade and balance on them i know sub frequencies become non directional below about 80Hz but I was thinking that the 8" would play into that range maybe between 60 and 120 or so Thanks guys in advance.