Welcome to our forum nimit1785.
A 20 gallon is a nice size for any of those species of fish and they can all be kept together. As a general practice, it is best to keep 2 or 3 females for each male of a species, when there are females in the tank. They can also be kept in male only tanks with no problems. In your 20 gallon you will be limited to about 10 of these fish in the long term so you will need to decide which ones to keep. For a beginner fish keeper I would suggest a nice group of 4 platies in whatever color you prefer. Platies are somewhat easier care than swordtails or mollies. If you provide good cover you will soon have plenty of platies to fill the tank.