The major balance problem with cults is that they recruit faster than you can lynch them. Typically, therefore, the only way to win is to lynch the leader, but that becomes harder as the game goes on because the other cultists clearly won't let you get away with that if they can help it. (Note that lynching a
cultist
, other than the leader, doesn't really help town at all when a standard cult is in use; their only real advantage is from the information they gain from the flip.)
In order to fix this, therefore, you need to slow down the rate at which the cult recruits so that the town can keep pace with the cult. Another bonus is that if you do this, you can sometimes make it so that there isn't a single vulnerable cult leader, thus making things a lot less swingy.
As an idea of the sort of things you should be looking at, here's a suggestion (aimed at 18-22 player games like the one you suggest): make a cult which can't recruit if it recruited a member the previous night, and can't recruit if a cultist died that night or the previous night, but which doesn't have a specific leader (i.e. any member can recruit); and also ensure that the playerlist as a whole knows about the presence of the cult and the rules governing it. Town can thus beat a cult of any size by accurately lynching it once every two nights.
As a side note, games including a cult typically shouldn't have other major anti-town factions (as this causes very large kingmaker problems as each faction needs to choose which opposing faction it wants to work on eliminating first), but if you do include other anti-town players (either factions or self-aligned players like Serial Killers), they should be completely immune to recruitment, otherwise Bad Things tend to happen to your setup's balance.
