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.