I never looked into the Gen 8 Project's Box link. Simply wasn't a big priority. as for a 7 party limit, I chose not to include it because of how the UI would look. I've uploaded patch 4.0.3 with something that I hope fixes the box link problem.It seems, that your Script doesn't work correct with the Gen8Project...
What I've tried so far, while both scripts are installed, is accessing the Pokémon Box from the party menu, via "Pokémon Box Link". This is a feature added in the "Gen 8 Project" and can be accessed by pressing the B key if you have the "Pokémon Link Box" item. Unfortunately that didn't work, so I uninstalled your script first. Hope this conflict will be fixed in a future version.
On a different note, why are certain party sizes not available? It would have been enough for me to have 7 instead of 6 Pokémon in the team, but the 7 is supposedly not supposed to work. Or am I just misunderstanding something?