So far here's the set up:
-Creepers are enemy Demomen
-Endermen are enemy Engies
-Skeletons are enemy Snipers
-Spiders (and Cave Spiders) are Enemy Scouts
-Zombies are enemy Heavies
-Blazes are enemy Pyros
-Ghasts are enemy Soldiers
-Zombie Pigmen are enemy Spies
-Pigs are ally Medics
-Butchers are ally Soldiers
-Farmers are ally Scouts
-Librarians are ally Engies
-Priests are ally Snipers
-Smiths are ally Pyros
-Villagers are ally Demomen
-Iron Golems are ally Heavies

Tips/Suggestions are appreciated

Additional Notes

I have based all of my choices of characters based off of the Mob's behavior.
Creepers are Demo-Men because they explode stuff
Endermen are Engies because they can build stuff
Skeletons are Snipers because they shoot stuff
Spiders are Scouts because they are quick and jumpy
Zombies are Heavies because they are big (Sorry, not much in the line of Heavy similarities)
Ghasts are Soldiers because Soldiers have rocketlaunchers, kind of like Ghast's fireball
Blazes are Pyros because come on, fire
Zombie Pig Men are Spies because they wield knives and don't attack right away
Pigs are your team's Medics because they give you health