Sounds like a good plan, if there's front and rear pre-outs, check whether the rear pre-outs can be changed to subwoofer pre-outs, i know certain Pioneer HUs have that feature.

Rear speakers powered by the HU isn't that bad, but try to fade your Front/Rear balance to the front so as not to create bad imaging(with the rears being overpowered and sound seemed to be coming from the back)

Hm... what you mean is that your HU only has a single pre-out? You'll probably need to get an amp that's have got a built in y-spliter, most reputable brands have the ability to only use a single RCA input for 4-channels, just have to take note of that when purchasing. It's rather not advisable to get a Y-splitter.

I second the Y-Spilter > Hi-Lo Convertor. Most Hi-Lo convertors, especially the cheap ones, would most often than not, induce noise into the system instead.

What's your budget for amplifier? If you are willing to get used, i believe you can still get used amps with the ability to use only a pair of inputs for 4-channel amplification.