I have always used the monitor outputs (CR--Control Room) with TRS 1/4" plugs out to XLRs on the other end into my monitors, and I have the stereo mix going through those. The advantage here is you will have a separate volume knob (CR) for your monitors, allowing you to adjust your input faders and your stereo fader all the way to unity, 0db, while keeping your monitors at a comfortable listening level. I think of the XLR outs more as a FOH output for use in a live setting.
BTW, glad it's starting to come together for you. You obviously don't have to use FW channels 31/32, that suggestion was just for convenience. 15/16 is no different at all. You can use any pair of channels you like, although most DAWs seem to prefer a stereo pair that starts with odd numbers...1/2, 27/28, etc.
One other note,
If you want to send your ITB stereo mix out to some outboard gear, I have found that it works better to send the actual FW OUTPUT stereo pair that you have assigned in your DAW, that is going to your mixer, directly out to the gear to be used, rather than sending it to a pair of DM channels, and then out through the stereo buss, then out to the gear.
For example, Say you want to send your ITB mix out to an external comp. Instead of going: 2 channels out of the DAW to a paired set of channels in the DM, then out through the stereo buss, and out through the assign sends and into the comp/out of the comp, through assign receive to a stereo pair of FW channels back to your DAW, try this:
Assign 2 FW channels with your STEREO mix out from your DAW (say SLOT1-1 and SLOT1-2) sent directly to Assn Send via your routing page, out to your comp, then back into assgn receive. Now set those Assin receives to SLOT1-15/SLOT1-16--in your case--) back into a stereo track in your DAW. Doing this keeps the original signal level the same throughout, avoiding any issues with routing through a paired set of faders and the stereo buss.
You should also assign the same pair of Assign receives to a pair of DM channels to monitor what your comp is doing. This way is best as the post comp signal is sent to BOTH to the FW directly and to a pair of DM channels, without fader volume affecting the signal being sent back out to the DAW.