Inkedup,
First of all, I need to make sure you understand the differences between component speakers, coaxials, etc.
Component speakers are sets which have separate woofer and tweeter drivers, and in some cases, a separate midrange driver also, all run by a separate outboard crossover network.
Coaxials are two, three, and sometimes even four-way speakers, but all within the same ‘unit’ if you will. In other words, these speakers would have a separate woofer and tweeter for example, but they are physically attached and do not use a separate crossover network. NOTE There are a few companies that make a supposed ‘component’ set in which the tweeter is attached but that still uses a separate crossover.
The stock speakers that our 'Tegs come with are just a standard speaker with a ‘Whizzer cone’ in the center that helps to produce and focus higher frequencies in place of a standalone tweeter.
Now that we’ve cleared that up…
First of all, something important to remember is that you always want your best speakers up front. This is where the soundstage is created. When was the last time you went to a concert and had the band behind you? It’s fine to have rear speakers (for rear fill, or backseat passengers) but it is far less important than building a good soundstage up front.
The way to install the system is that you place your best pair of speakers in the front. A coaxial is easy, because it simply replaces your stockers. If you have a component set (which I highly recommend if you want clean sound), then a more complicated install is required, but is not difficult.
You would install the woofer in the stock door location, and then find a place to locate the tweeter while keeping it as close as possible to the woofer it goes with. This is VERY important. You would NEVER want to put the woofer from a component set in the back, and put the matching tweeter in the front. For my installs, I always just cut a small hole in the door panel above and slightly forward of the door handle and window switches and install the tweeter there. This provides excellent staging and imaging.
With regard to rear speakers, I’ve never found it to be worth investing in component speakers for rear fill. Typically, in my systems, I don’t even have rear speakers. My favorite G2 system to date is the previously mentioned Focal 165w’s in the front, and a single Boston Acoustics Pro-Series 12" sub (dialed very low; I’m not into booming massive bass, although this unit is capable of it!) in the rear hatch. Properly powered, this will give you unbelieveable sound quality. If you must have rear speakers, I would HIGHLY recommend just getting a coaxial set that matches the front speakers and throw them in the stock location of the rears.
I hope this all makes sense, but if you have any other questions about this (or if any of you G2IC-ers have questions for me) please feel free to ask. I spent a few years in the car-audio industry, as well as the home audio industry, and I have a good base of knowledge (especially about what SOUNDS good, not just what has the loudest bass).
HTH