We really need Larian to limit party size to max 4 players with the ability to have 2 origin characters traveling with. For multiplayer purposes, 6 makes sense.
So, to fix balance issues, single player should also allow 6. Main+5. Then, if the game is balanced around 6, it works every time whether 4 players or 1, you can still have at least 2 origin characters in the party at all times for story content purposes.