The problem you have is that mollies and white cloud minnows are schooling fish, which need at least 3 (and preferably 5 or more) fellow members of their species to be comfortable. The problems you have named are common for when you have two few of a schooling fish. If the fish you listed above are all you have in that tank, and it is a 20 gallon long, you could get away with adding 3 or so more mollies, 2 or 3 more minnows, and maybe another black skirt or otto while you're at it, since those are both schooling fish, as well. If your tank is a 20 gallon tall, then that would likely be overcrowding the tank a bit, and you should instead think about finding a new home for either the molly or the minnows, and getting more of the remaining fish. In fact, you might even consider doing that anyway, even if the tank is a 20 gallon long--you just have too many kinds of schooling fish in too small of a tank to comfortable house decent size schools for all of them.