Hi what a subject,
And that’s it its all very subjective you wont get two sound engineers with the same ideas or totally agreeing on this one.
When to Normalise when to use a compressor. When to work on the whole mix or a track by track instrument basis.
I guess it depends on how well you balanced the original tracks and final mix of the whole.

The less after work required the better in my opinion. But if at the end of day you finish up with a good sounding mix at a decent level above the noise floor except that you have a kick drum or brass instrument that’s a bit aggressive you will probably get away with compression just to tame the beast, but then again, if you had taken more care on the offending instrument at the time (assuming recorded separately) maybe you should have compressed it then if it was say a drum sample the kick may have been a bit lively before you started.
Of course compression is a subject on its own and can be level and or frequency conscious as well.

Where as normalise is more to do with levels, again maybe at individual track level or the final recording.
You have to be careful not to overdo any of this especially when working at track instrument level, mind you don’t remove you intended expression and feeling that you played in . or you can finish up with a bland over sanitised sound.

Here is a chart to show the musical equivelents of midi keyboard velocity or expression 0-127
It will at least remind you that you can have different levels on tracks.

An extremely good idea if we could map these concepts of loudness to MIDI note velocities. Lo and behold it has already been done. In XGworks, the first "proper" sequencing program I ever used, it uses ppp through fff against the note velocities in both the "staff view" and the "piano roll" windows and also within "list view" window too.

It works like this:
fff = 120.
ff = 104.
f = 88.
mf = 72.
mp = 56.
p = 40.
pp = 24.
ppp = 8.

These are, in fact, centre values, so for instance ppp is from 1 through 8 to 15, pp is 16 through 24 to 31, and so on.

Have fun
Mike

BIAB2021 UltraPlus,AsusN55S1Tbssd, W10/64,Akai EIEpro
Yamaha CVP405,SquireStrat, CoolsoftVMidSynth
Novatation Impulse61 Ctr kbd, Cwalk blab Kontakt

http://mikesmusic.byethost16.com/