Helix input, and rear fill frequency

Before installing the Helix.3 DSP I had run three sets of rca cables to the trunk, front, rear, and sub to get the signal to the amps.
After installing the DSP when I first set it up, I used two sets, front, and rear.
I am now setting up a three way front, and rear door speakers will be mono rear fill.
For input to the helix, is there any reason to use front, and rear? I was thinking just using the front channels would be all that is needed.
Also, the rear door speakers are 6.5" Morel Tempo Ultra that were up front in the doors playing from 80 to 3200hz in stereo.
With the new setup the rear speakers will be mono.
The front mid base door speakers will be Stereo Integrity TM8, with a recommended crossover point of 55hz. There is a pair of Stereo Integrity SQL-12 in the trunk that they will be crossed over with.
What would be a recommended crossover point for the rear fill speakers?
Thanks for any help.