That's how most are actually designed to work....some 5 channels act more like two separate amps under one chassis incorporating a class a/b design for the 4 channels, and a class D design for the sub channel....the Memphis Belle and the JL 500/5 are two good examples of such a hybrid design. You will have no problems mixing loads on your channels no matter what 5 channel you get.
It seems cost is a factor in your application, so why not get a more powerful 4 channel (75 or 100 x 4) you can the bridge it up to run the rear channels to the sub, and get more power to the front channels...
I have never been a fan of rear speakers, especially if you have a sub...spend the money saved on expensive rears on getting a nicer set of components up front, and you can power the stock rears from your head unit.
I find this type of setup to be more cost effective and to actually sound better then the typical 5 channel.....just a suggestion.