I FINALLY GOT IT TO WORK... but then I had to stop playing for the day.
BEHOLD: PARTY OF 6!!! TAV + ALL ORIGIN CHARACTERS
So, first you have to use the ExtratTool-v1.15.10 as instructed in the Mod instructions. Switch Max Party Size from 4 to 6 or 8. Save. I even used the Extract function and chose the same file folder as the Save I was using. Not sure that's necessary, but I did it anyway.
But here was the step that I had to do to make it work in the end. I loaded up the modified save file, and they still wouldn't let me add more than a party of 4. So I dismissed everyone from the party and reinvited everyone back into the party. THAT'S what made it work. Once I reinvited everyone, suddenly, on the 5th person, VIOLA! No more, "You're full up."
Now I just need to test combat with it. Looks like I'll be starting over my 4 Custom Character game to try it out... or I may just keep going with Tav and the gang! Yeah. I think I'll do that. Then I don't have to dismiss or pull in any characters at all for story content.