You DO need a heater with your goldfish to at least regulate the temperature. Just make sure it is set on low. That way it doesn't have drastic highs and lows.
If you get a fancy goldfish, I wouldn't suggest a betta- now, all fish are different and they might get along but the long flowy tail of the goldfish might confuse the betta into thinking its another male.
White Clouds are a good idea.
Also, make sure you have around 10gal of water for each goldfish- they are messy creatures. And if your tank permits you can always get different colors and breeds of goldfish to make your tank not so... monotone? heh