Would a teleport to the back be enough, just to make space for the next pair or does CharacterAddToCharacterCreation( _Char, 0, 1 ) require region change ?
Unfortunately it does take a full blown region change. So you get a load screen for Cyseal and then another one immediately shipping you back to creation. You never actually see Cyseal though you might hear the sound effects for a couple seconds.
Think I almost forgot that all companion stuff would have to be adjusted as well because without change there would be two additional companions to hire on top.
In my mod I treat the #3 and #4 players as companions. I actually run them through the MakeCompanion procedure and add them to the relevant databases. So you don't have to adjust for 4 characters. You would have to actually increase the CompMax number in a six character mod as the problem would actually be sort of the opposite. If you dismissed #5 or #6 you wouldn't be able to rehire them.
If you made them true players and not companions, then yeah it would require reducing the companion number to maintain 6.
