Are these ok in my 20 gallon aquarium?

Yes, there are to many fish in your small aquarium. There should be atleast 1 gallon of water for every 1 inch of fish at their full growth. Silver dollars alone can get up to 12 inches. I would try to get a bigger aquarium and put the silver dollars and gouramis in their and find a new home for the betta or a bowl. You are definatly overcrowded and the water quality is going to go down big time and their wont be enough oxygen for the fish to live.