I read somewhere that fungus could not spread to other fish but learned in school that by reproducing they make thousands of spores that float around until they find something to land on. I also read that to get fungus a fish has to be weakened meaning it could have had a disease and is temperarily weakened, or fungus showed up on a scratch or scrape that hasn't healed. Check your water quality. I'm not sure but if your molly is healthy then you could probably put the pregnant fish in with the fungus fish UNLESS your doing a treatment for the fungus. If you are treating it then the next time you go to the store buy a breeding net for just this once or if you have another tank, use it.
