I'd say that the mollies are definately pregnant. It's very easy to tell the gender apart from mollies. Males have larger fins especially on top of their body. Females are larger in size and have smaller fins. I'd say, especially because you have so many mollies that most are pregnant (there would have to be at least one male in the tank!). You will soon probably have a lot more than 21 mollies!
I have a similar question. I have had 2 white mollies for about 2 months now. I was told they are both female but one of them seems to get pregnant. I noticed that her belly was very large and she was having trouble swimming. A few days later all of the bloating was gone and she was swimming fine. Now I have noticed the same bloating again on her left side and difficulty swimming. Is she pregnant? I have separated her in a fry tank along with a pregnant guppy that I have just in case...