You want your amp(s) RMS rating to meet or exceed the RMS rating of the speakers or closely match. Then turn down the gain of the amp so you do not get distortion. If you buy smaller amp then you may turn the gain up on the amp for more volume and end up sending a clipped signal to the speakers. Ignore max rating and look at RMS.
I personally prefer to buy a quality set of front components, amp them, and not run rear fill. Good quality front speakers will provide more than enough volume and sound better then 4 medicore speakers. Only reason to run rear speakers in a vehicle is 5.1 surround sound with separate rear channels for movies. I havent run rear fill in 10 years and you would never know.
