I say go for it man, but make your front'* dominate. You need the sound stage forward.
I've got my old componets in the rear deck, mostly faded in just for passangers but lately I have it setup 10 out of 15 towards the front'* so little comes from the back, but don't want to hear any voices though.
Although 6x9'* will fill the gap nicely between components up front and subs in the rear... You'll have a good 20-30hz gap...
What gap are you talking about, most setups dont go to 20hz? That'* pretty low, Mines not that low.