You're definitely on the right
track (pun intended) by using the low voice/yelling/high tone tracks
each in their own way.
What you're talking about sounds like a mixing thing to me.
Try this-
Have your
main vocal track in the center of the stereo image (panned 0%) and summed to mono (Usually there's a knob for this)
High pass filter that mf'er at 100 hz using any ol' built in EQ plugin
Then for the side vocals- high pass them a bit higher, like 200-300 hz, also summed to mono, and obviously panned away from the main/center vocal
That should mostly get them out of the way of the main vocal. Especially if you cut (yes, cut) the
main vocal where people typically sound nasally (600-1000 hz) and boost the main vocal's presence around 3.5-4 khz (like 2-3 Db)
That gives the main vocal a beneficial dominating frequency range while cutting out the bullshit (where your side vocals can come in and shine).
Regarding the "fullness", I'd download the free ADT (stands for Artificial Double Tracking, i believe) plugin that I've linked below- the link that says "vacuumsound."
(( vacuumsound ))
It's a great tool that you can fool around with on your vocal tracks (especially overdubs) to make them sound "fuller".
Best of luck!