Yes, you can turn down gain settings for the rears.
I have 800 watts going to each of my 10' subs. Rated RMS on them is about 200 watts, I believe. The idea is to have the headroom available and adjust the gains so that there is absolutely no distortion.
The problem is see with your setup is that you want to do a single 10" sub in the trunk. While it would be ok to complement a stock system or even an aftermarket system powered by a head unit, you will not be able to hear it if you amplify interior speakers. Unfortunately, our cars are very bad for bass, so you need to go a bit farther than any other car to get decent bass.