i installed a JVC CD MP3 player and used the front pre-outs only, no speaker outputs. It works great. The stock Inifinity system uses an amplifier that accepts left and right signal only (no front or rear). the amplifier then splits the signal for the tweeter, mid, and subwoofer. I have had previous stock system systems (Bose, JBL, etc.) that used a speaker level input to their amplifiers. The Infinity system in the XF is not speaker level.

As far as adding rear fill speakers, now that you essentially have an unused amplifier in your new head unit, you could run those wires throughout the interior and theoretically add four more speakers.