Betta living with goldfish?

Not a good idea. 76 degrees is too warm for goldfish (and too cool for Bettas, btw). Goldfish and Bettas are not compatible. If you get a 5 gallon tank with hood, heater, and gentle filter, you could cycle it for your Betta. In the meantime, you will have a hard time with the goldfish. If they are indeed babies (less than 2-3" long), you might transfer them to the tank until you can get the Betta out of there. The 20 gallon is only a temporary solution. As you already know, Comets get huge. You will eventually need a 100+ gallon tank for them. Alternately, you could be looking for someone who has an ornamental pond for fish. When they large enough to go in a pond, you are all set.