I thought I posted a reply yesterday, oh well.
First be careful with terms, you are not MIXING in Compressor. That is something that is done in an audio program, in this case Pro Tools. you are just assigning the stems to the correct channel.
Each stem needs to be a mono stream that is panned to the center. Then you must be using a 5.1 preset to set each stem. Then your reciever must be set up correctly to play out 5.1 and not to a stereo downconvert.
You also never use FCP for something like this. Unless you go in and change the way that audio layers are outputed by default everything goes to ch 1 and 2