Hey Anonymous, I wouldn't downgrade the speakers to match the amp. The rms power for the amp is at a normal setting, if you turn up the bass, it'll go past the 50W rating. Personally I think your rear speakers are too small. Why do you have 5.25"? Why not 6x9"? Just curious. One more thing, I would buy a stronger amp for the system. I find that when you turn up the volume, a stronger amp will produce cleaner sound than for example driving a weaker amp to it's limit.
Personally, I'd forget rear fill entirely and focus my money on a good front component system. That would require seperate amps for the speakers and subs, though. But, if you want rears, that amp will be fine for them. You'll be perfectly fine powerwise as long as you don't drive the amp into clipping. As far as the subwoofer, personally I think you're better off getting the DVC 4 ohm version of the 12.1 and put that on the amp, that way you'll get 400W RMS to the sub.
Driving the amp into clipping...at what volume do you think that would happen? I don't plan on being heard a mile away I just would like some decently loud tunes when I wanted to hear some.
If you set your gain correctly, it's usually at about 75% volume or a little above. It just depends on where you set the input level. The gain setting will be totally dependant on your head unit preout output, and also the reference material you use (preferably a 0db sine wave). A voltmeter or oscilloscope are the best ways to set gain, going by ear is the most typical, but setting gain for subs can be a little harder to do by ear. With a decent head unit, gain typically is set to half or less.