I have the following so far:
Alpine CDA-9807 head unit
Alpine MRP-M350 mono amp
Alpine 10" Type R DVC
Infinity Reference 605cs 6.5" components

Now my car has 4x6 up front and 6.5" in the rear. I plan
to put the Infinitys up front (6.5" on door, tweets in
kick paenl), and the rear speakers I will just disable.

Question is, should I get a 2-channel amp to power the
components, or should I look into a 4-channel, in case
I decide to put a pair of 6.5" 2-way coaxials (Infinity 652i)
in the rear?

It seems front stage is more important and the sub will just
cancel out the rear, so right now I plan just to disable the
rear speakers all together.

What should I get?

 

Go with 4 channel. For the cost difference, even if you keep the backs factory it's best if the deck amp is not in use. Makes your fronts sound that much cleaner, less heat...

 

Well I don't like rear fill anyways so I would say get a nice big 2 channel and run the components with lots of clean, unclipped power.
Or you could get a big 4 channel amp and put the component set in the kicks and some midbass speakers in the doors.