Question about specs/replacing factory speakers and head unit on 2000 4Runner.

I called Crutchfield and forgot to ask if I could use component speakers as there is a factory component tweeter at the front of the side windows. He also said I would use 6 1/2" speakers for the front doors, but could not use them in the back doors and should go with 5 1/4". Haven't decided on an amp, is it pretty much a given I will need one with component speakers? I am leaning towards a Kenwood receiveer with either Infinity, Alpine or Pioneer speakers. No sub for now, so want door speakers with best bass possible. Any advice/opinions will be greatly appreciated.