Archerfish are brakish water fishes so they should be kept in brackish water. I've never kept them myself, but Baensch/Rhiel say they need salt in the water, and there might be some aggression which suggests to me that they might not get along with other fish they could eat.
Sounds like they are in with mollies, if they are eating the young; mollies can adapt to brackish water.
Moving them into freshwater will stress them as it is not natural for them, then they will be prone to disease. They`ll certainly be swimming around less if you do that.