The -1.5dB indication on the masterbus fader is the gain
applied at that point- which is simply a relative measure of the gain change it is providing at that stage. Since the program is operating internally with floating-point math, that relative level change may or may not be appropriate for preventing overs in the fixed, lower bit-depth file output.
Process depends somewhat on the software you are using. Most modern Digital Audio Workstation software works with floating-point mathmatics, which means you typically needn't worry about internal headroom or clipping
within the software, only on it's way in or out of the DAW. Typically you will have control over the signal level of each individual mono channel or stereo channel as well as separate level control over the 2-channel mix bus (maybe called the master bus). Since the program itself is working internally with higher floating-point bit-depth math, you don't really need to normalize the individual parts to be mixed, although doing so may can help manage things for you. What is more important is making sure that the final output level from the master bus is the correct level- not to high, not to low, but just right Goldilocks.
Basically, my "mix and master method" is:
Bring SBD to 0db
Bring AUD mics up to where I like the sound
Mix them down into one file where the resulting file is -2db (mainly to give headroom)
Or if straight AUD:
Bring AUD to 0db
Mix it down into one file where the resulting file is -2db (mainly to give headroom)
Just a reminder that the most appropriate "mix and master method" depends on the qualities of both your AUD source and SBD source. With a really good AUD, you'll probably do best with a lot more AUD than SBD in your matrix. Usually some SBD will be beneficial for even a fantastic AUD, but with a really good AUD often very little SBD is necessary- just enough to bring up whatever elements may be a bit low in the AUD, and to crisp-up vocal clarity and transients and bring those elements forward a bit to make then more immediate and engaging rather than soft and distant sounding. That's favoring the stew to taste by adding enough SBD spice to the base stock AUD.
If the SBD is better than the AUD then doing it other way around lie you mention may be more more appropriate- adding just enough AUD to bring the dry but better sounding SBD to life, with the AUD's room and audience contribution rounding things out. In that case you're adding enough AUD spice to the SBD base stock to taste.
So, no hard rule concerning which component should be higher in level or normalized first, play that part by ear. Just check to make sure that the final output bus level is where you want it before writing the output files.