Now I'm going to separate the roles into categories based how I think a reasonable FM host would separate them
Protection (Aiding in the survival of town): Armorsmith, Physician, Bus Driver, Operator, Coward
Offensive (These roles are town's offensive weapons): Gunsmith, Chemist, Electromaniac
Investigative (These roles help us learn info): Snitch, Tracker, Scout, Investigator
Vanilla (These roles either have no active ability or should not use their abilities at night): Citizen, Janitor, Tailor, Bartender
If you are a Janitor, Tailor, or Bartender, do NOT use your abilities, as they will confuse us.
Town is pretty simple to separate into its appropriate categories, standard FM balancing principles can be applied to a town's makeup.
Protection: Physician, Coward
Investigative: Scout, Tracker, Investigator
Disruptive: Janitor, Tailor, Bartender, Bus Driver
Offensive: Blacksmith (realistically the blacksmith is likely going to give guns to himself to increase kpn by .5), Electromaniac, Chemist.
I don't think we're likely to see duplicates in category for the scum team, no double investigative, no double protection, certainly no double offensive (although I doubt we have any offensive whatsoever as the kpn would be OUCH!), and unlikely double disruptive.
Now to look at the role distribution.
Two power scum, up to three town power.
Generally you want the TPR to match up with the amount of power scum roles, but not all roles are made equal.
If we have a janitor, this game gets a lot harder, as it appears they have unlimited charges, and it's not clear whether alignment is revealed.
If we live in a world where we have a janitor, I expect we'll have three town power roles,
if we don't have a janitor, then I expect we have two true TPRs and another role from the "vanilla" category I invented.
Here's my guesses as to role list for scum, and I'll keep my speculations regarding the town side to myself outside of broad generalizations of amount.
Investigative (likely tracker or investigator, as lookout abilities have significantly less value in the hands of scum outside of multiball)/Non-janitor disruptive vs Two true town power
Non-scout investigative/Janitor vs Three true town power
I find scum having a protection to be unlikely, protecting yourself from night kills as scum tends to lead to a reveal from the person who attempted to kill them, outing the scum in the process. If scum has a KPN we are toast, so I don't think martin gave them it (unless he was feeling particularly sadistic when choosing the list), so it's likely investigative and a disruptive.