They are cold water fish. I think though, if they had survived, you would have had to separate them from the goldfish as they get aggressive.
Fine white furry growth is often a sign of infection - often secondary, meaning it was a result of something else wrong.
Have you tested the water for pH, ammonia, nitrates and nitrites? Often, if fish are living in less than optimum conditions they adapt but any new fish quickly succumb to the poor water quality. I would not have added more fish to the tank.