Clownfish generally only have a small territory in a tank.IME,they only chase away other fish long enough for them to leave their therritory.They don't continue to chase fish around the tank until they die.In fact,they rarely will ever leave their small territory.Most of there aggrssion is targeted at other clownfish or fish percieved as a threat.Mandarins and Firefish are smaller,are differently colored,and have a much different body shape so I think they would co-exist peacefully.I think you will find that getting the 2 clowns to co-exist will be the biggest problem.