It is totally safe for the other fish. when I treated my platy I had 4 Zebra Danios, 4 Black Skirt Tetras, 2 Playts, an algae eater and a Pictus catfish. All were totally fine.
But before you add the melifix, remove the carbon from your filters or it will just filter the medicine right out of your tank.
If you run a cartrige filter, just make a little slice in the floss or spongy stuff and empty out the carbon. Good Luck!