Most passenger missions are for no more than 16 passengers per group, which is the capacity of a size 6 Business Class cabin or a size 5 Economy Class. I'd recommend Economy for most cabins smaller than 6, but make sure you have at least one better cabin for higher-class passengers.
My Python has three C6 Business, two C5 Economy, and one C4 Economy. That leaves two C3's for a shield and FSD Booster, and a DC in the smallest slot. That's for short runs where everyone is going to the same destination.
For longer runs to multiple destinations, I usually use my Beluga, I'm still adjusting the layout for that (the removal of the slot restrictions will help somewhat). Then I use the galmap to pick the order of destinations based on distance and direction.
I never take criminals on the Beluga (except by accident...), but I have a stealth-build DBX with a C4 Business cabin for those.