Well, for each kind of platy you have you could get 1 more female of each. It is the reccommended ratio for livebearers. So you 20g would look like this stocking wise:
1 Blue Paradise Gourami
3 Sunburst Mickey Mouse Platies(1 male 2 females)
3 Sunburst Dwarf Coral Platies(1 male 2 females)
5 of the schooling fish you have listed above
I think that is a perfect stocked 20g. Good Luck!