iirc modding the party size is trivial, merely a tweak of a text file (which shows that it is a deliberate choice by Larian).
Yes and no, there are areas that will bork up if you have more than 4 party members (one of this is/was the boat scene in the underdark). And I *think* you don't get the interaction with the addition slots filled, the game goes on as if they didnt exist... So yes trivial, but in some cases no. So in those cases, it would take additional modding as this could bork a game later on...
