You run three instances of the game in parallel for the custom characters, then use this save editing trick to expand the limit of your party up to 6 or even 8:
Also i noted that even tho UI is able to contain 8 companions ... its certainly not created for them, once you recruit your 7th character, party window dont get wider, but you get small up and down arrows to slide.
I still dont understand why cant we change Race for our hirelings. Lets us play Githyanki as racist as they trully are!