(especially when certain "roles" must be filled, e.g. Lockhart running both a fighter bot and an arcane bot in early Shattered Star)
Hmmm? Who did I run for arcane? I gave them Brunsk fairly early, but that's more so because I'm a softy and don't like killing PCs early level just because there aren't enough HP to go around. There was Natalya, but she was just an NPC that could be convinced for help, or asked to stay back and I think I played her more like a rogue, she really isn't that interesting magic wise. That was just part of the adventure, as are other NPCs that occasionally join in for short periods of time.
Regarding number of players, I've just tended to like smaller, more intimate groups myself of late. Maybe that's just because one of my funnest and longest running groups has been just me and three players. It keeps things simple and quick to schedule and play, everybody can get their turn without being left out, and in general, with experienced players, three honestly is enough to do most of the PF APs that I like to run, specific hard encounters aside. Honestly, early levels aside, I'm not a big believer that PF has roles that must be filled for success. Once you get past level three, affordable magic items and cooperation can make most party combinations viable, if some more difficult than others.
But this is getting a bit away from RM. Which there has been discussions of.