I would only take the Loonboss on Mangler as your general if you plan on going full squig with multiple hopper units (I sure will be!). The bonuses you receive for using Skragrott the Loonking as general is otherwise too strong to pass up for a lesser leader in my opinion, not really worth using the Manglerboss for 1 unit of hoppers, also note Manglerboss command ability can be used without needing to be your general. So really depends how you want the core of your army to be.