The platy and gourami are ok but I wouldn't do the molly. Mollies get rather large, combined with their bioload they should be in something like a 30 gallon IMO.
I'd try only a single DG, trying 2 in there probably won't work as they can get pretty nasty with each other.
What do you have for filtration?
Depending on your filtration and if you just kept the DG and platy I think you could probably do about between 7 and 10 platy. If you are doing males and females than I would suggest 2-3 F to 1 M. SO like 6 F and 2 male.