The spotted molly is called a Dalmatian Molly. I have one and a Gold Doubloon molly too. The Dalmatian molly chases the Gold Doubloon molly a lot. They chase each other and seem to play. At feeding time the DM gets a bit rough with the GDM so I feed them on separate sides of the tank. When the DM gets bratty, the GDM will hide in the plants. They have never hurt each other. I am hoping once they get some tank mates they will chill a bit.
You can tell a male from a female by their bottom fins. The female has a rounded bottom fin right in front of her tail. The male has a long, skinny bottom fin. Somewhere on these boards there is a very good picture of a male and female but I couldn't find it right now. If I find it I will post a link.