Well, being a former mod, taking on extra roles after a theme has been set is a pain in the...
...oh, crap, there's already mods in this thread. I can't say that.
But anyway...
...it can throw the balance a bit. The last game I modded was crazy not only because of the amount of roles, but the amount of townies just made the game drag for too long. I do agree that 17 roles is a bit slim and that expanding it to 19 or 21 would be a welcome addition. however, I don't know what role set FLLM is using, so a 21-person town could throw things off hugely.
However, if a certain role does exist (Anthony, I'll tell you which one I'm thinking of), then he probably should expand the town to make things balance. If not, this could become a speed town. And no one likes a fast village.