To your second question...
"...he has the multi-timbral parts all within one track and one region after splitting the notes to midi channels. Is it still the same to then send each instrument/midi channel to its own aux in the mixer?"
Yes, it's the same.
Generally there are two approaches to using plugin instruments when it comes to creating a multi-timbral arrangement. One is to host multiple instruments in one plugin, and the other is to use separate tracks. Let's use the 3-voice sax section as an example for separate tracks (which is what I believe you're doing):
Three tracks, each assigned to its own instrument, each hosting a different plugin, where:
- track 1 = alto (Kontakt)
- track 2 = tenor (EXS-24)
- track 3 = bari (WIVI)
With this approach you have:
- individual control over the volume of each instrument (via the instrument's channel strip faders)
- each instrument can have its own unique combination of plugins, sends, panning, etc., all set right on the individual instruments' channel strips
- each instrument's region can have its own independent staff style as well as settings for the realtime parameters (transpose, velocity, etc.)
This is by far the most straight-forward method of creating an arrangement. Everything is completely independent, and personally I prefer this method. But there are times when the other approach of using multi-timbral plugins (and all of their associated complications, such as creating auxes to handle individual outputs) has its benefits.