Why you dont need any rear fill
I took mine rear ones out from day one and have had a nice set of components up front. Currently my amp is dead though cus I was um making it look nice and shiney, and wasnt thinking about the inner voltage regulators having different grounding points when I put it back together..... I released the magic smoke.

Oh well I tried to fix it with new parts (soldered in new parts that where physically damaged) but its a no go, gotta send it back.
For my rears I may be adding a sub stage again after I ripped my old one out for lightness. Im stuffing a set of boston acoustics pro 8"s in there with fiberglass boxes mated under the rear deck or putting them in the rear quarters
BTW my car sounds (err sounded) very nice (weeks of tweaking will do that though) with just the front set of coustom mounted BA pro 6.5 components.
Didnt really help eh? hehe
Oh to fit 6x9's I would use a jig saw with a fine metal blade, cover the gas tank area with a drop cloth and ,cut it out of both sides of the speaker hole to keep the speaker centered. Test fit them then mark the mounting holes. Drill those out and drop them in / wire them up.