Your mollies will likely get too big for a 20g, I would rehome them if you can. I probably wouldn't add any females with your current stock, you'd just get overrun with babies. I'd say without the mollies you're pretty well stocked imo.
Agreed with the above post. Your mollies will outgrow your tank pretty quickly. Adding females to the tank will only lead to an even bigger biolad in the tank and a quick overpopulation of fry, possibly cross breeds of guppies and mollies. If you rehome the mollies, your tank will be at a good stock count.
Speaking from experience, I currently have about 50 fry in a 10 gallon tank and they are mostly from my mollies. I thought I had all females but upon closer inspection found two males that I gave away since then. Luckily my locally owned pet store will take them once they're big enough.
The most common molly is the black form of P. sphenops, which does not get too big for a 20 gallon tank. Sailfin species/hybrids will get much larger, and would be severely crowded. So the first question should have been, what kind of Mollies do you have?
The problem of livebearers having a lot of fry is another matter, and is a future concern. If one has all males it is not a problem, but as soon as you put a female livebearer in the tank, you will start getting babies in relatively short order.
I would be more concerned with the Tiger Barbs, as they need a lot more swimming room than any of the livebearers. If I placed 6 Tiger Barbs in a 20 gallon tank, that is the only fish I would have in there. And it would be a long 20, not a high 20.