Building New System....

I am pretty set on my sub and amp system just purchased, just waiting for it

1 - ported box, 2 - Punch HX2 RFD2212 12", 1 - Rockford Fosgate 851s Amp.

But I am still not sure on my interior.  My care accepts 61/2" in front and rear deck.  Bummer on the rear!  So I was thinking about Infinity Kappa's Components for front and back. 90w each.  Easy to match an Amp to that!

But then I was looking at the Kappa 6x9's and some exterior boxes to put them in for the rear.  They take 110w.

My question is this...can I get one amp to power both???  If I get a 4-channel amp that has 90 watts per channel.  Would that be okay for the 6x9's?

Or if I get a 4-channel that has 100 or 110 watts per channel, would that be good if I turn the gain down for the 61/2's????  Would that much overpowering the 90w speakers really be that bad?

I am open to suggestions!