Obviously one male and one female is needed, but also the temperature of your water must be warm enough as we are talking about tropical fish. Different types of molly can breed, so can different species from their family. Guppy/molly hybrids (gullies) are an example.
Yes, a black Molly is a breed of fish in the breed "Molly". They grow up to about two and three-fourths inches from head to tail. These fish are a community breed so you can mix them with other community fish. They come from Central America.