I've been able to so far successfully add the 6 origin characters to one party (playing as Fane). Everyone follows me correctly, but I have no idea if this breaks any other scripting/dialogues/combat/etc.
Screenshot of 6 person party: https://i.imgur.com/QwMwfjr.jpg
Steam workshop in case anyone wants to test: http://steamcommunity.com/sharedfiles/filedetails/?id=1137971468