I am agreeing with Bart, (mostly, I wouldn't pay ZERO attention. Watts are like HP, it's what sells so it has a relevance to what you are paying for.). So hopefully, this build on his comments while answering some of your questions.
There is a certain amount of hype on speaker Watts, and amp Watts; RMS and peak have technical definitions but there is a certain amount of marketing around this. My Polk recommendation is simply what I use and fairly affordable. Fosgate is probably a notch or two higher, at least.
Yes, you can have an amp with more power than speakers and be fine. Impedance is really the thing that matches what the amp delivers to what the speaker can take. Bart is right, it's true the cleaner the signal the better. But a great amp with an over driven input can distort anyway. Modern high quality stuff has protection circuits (auto gain control, or a feedback loop). Matching brands is going to help a lot because presumably this stuff is designed to work together and they avoid warranty issues and general customer complaints.
Keeping the speakers rated higher gives some safety margin, especially with mid to lower end stuff. A high power amp can "blow" a low power speaker by overheating the coil and shorting it out (never seen it) or by overcoming what the acoustic suspension can handle. The latter is what we typically think of as a blown speaker. The former would likely take sustained high power operation -- heating. The later like more to do with the signal quality and volume (significant bass, distortion) that move the cone significantly. And probable helped by fatigue over time. Keeping the volume down makes all of this unlikely.
It's unlikely you would notice a difference with a higher power rated speaker, like the 100 W you asked about. I wouldn't be a bit worried about 100 W speakers with your 45 W unit. It's typically one speaker unit per channel that you are comparing. A mismatch is not going to be a problem unless you have a really low power head unit and the big speaker, something like a woofer that has big mass. It would work, but you would need to turn the volume up more. And maybe not much more. This assumes your really low power radio would actually sound good, which it probably wouldn't all other things considered.
Bart is right, you don't need rear speakers (even though he does have one
). 6x9's package well and being in the package tray they benefit from the space in the trunk for low end bass. You don't need them with a sub in a cabinet or a sub in the package tray. You may want them if you don't. A better place for rear speakers is in the door so they aren't blasting behind the head of a rear seat passenger. This is how the OEMs do it; a sub in the rear tray and a rear door speaker. This is hard in a two door, no door and putting them above the armrest is is the way of the glass. The reason mine are powered by the head unit, is that the amp that drives my front speakers is set up so that the fronts are louder than the rears at a given setting on the dial.