While the 500 roof racks probably fit the 500c, the c has no specified roof load (at least in Germany...I hear different messages from Austria). So this might be borderline illegal.
Same with tow bar -the 500c has no defined tow load in the papers. That might still allow the tow bar for a bike rack where it does not pull anything...
A towbar with a removable hook is probably the most stable, unobtrusive and practical solution.