I played with a 6 party mod and it worked great right up until the point it failed to launch the grove party. The game is already set up to accommodate 6 party members.
There were some things that would need to fixed
-- party management. A couple of times my toons did a little dance trying to find where to stand - the default party formation was the bowling pin one - party inventory, with six people the items got so small I had a hard time seeing them - minor graphic glitch during conversations where the extra two party members merge into another NPC
But I got what I wanted, more banters / conversations and it felt a bit more like Baldur's Gate.