Non-convertable scum sounds good. However, there's still the problem of scum-turned-survivor. If scum lose their member they'll either: a) know that he has been converted to survivor if we explicitly inform them about it or b) will have a survivor in their ranks who they think is scum if we don't. In any case, and especially in second scenario, the turned guy will be able to convert all his former teammates to survivor and quicken his own victory. However, in the first scenario, scum will be able to immediatly kill the turned guy. This makes any conversion out of scum deadly to at least one player. Is that an optimal situation?
Also, what will freshly-made level 2 do after being 'killed'? Will he immediatly claim and get lynched, turning him into level 1 to deny the scum a potential recruit?
And one final thing. If scum die by getting lynched, there's no point in raise-by-two-levels ability - it adds more risk (level 1 turning into scum), but no additional profit.
Meanwhile, during the last night I've thought of an alternate version of this setup that completely scraps town/scum division:
5 levels, each starts with two players, everyone's goal is to get over half of living players into his level. Additionally level 1 has a survivor wincon. Levels -1, 0 and 6 kill. Everyone except level 1ers know their teammates. If a level has an ability, everyone on the level can separately use this ability in the same night on different people. Nobody can do anything to his teammates.
Level 1: No abilities.
Level 2: Can uplevel by one.
Level 3: Can inspect.
Level 4: Can downlevel by one.
Level 5: Can downlevel by two.
During the day, everyone can vote either to bring someone up one level or down one level. To prevent everyone fullclaiming and congregating into the same team, a rule: if a player gets converted into the team that wins on the night of his conversion due to his conversion, he loses instead.