Each amp is designed for a specific number of speakers, so your 4 channel amp is designed for max 4 individual speaker channels. If you want to power 6 channels you can use 1 channel for multiple speakers, but like your installer was saying, that can be bad for your setup, speakers and amp.
If you want good sound start by ditching the door subs, then use the amp you have to power all 4 main speakers. Buy another sub specific amp for your subwoofer and you will like that better too.
If all you want is the cool factor then you are doing it right... if you want something that sounds good you will want to listen to your installer or do some research yourself before buying anything else.
If you are just venting that is fine as well, and feel free to ask questions so you can understand what your installer is telling your. you might even be able to install some of this stuff yourself and save you some money.
I hope that helps, and like I said ask away and Ill help explain and answer anything I can.