Fishy question!!

You should be able to put the betta and goldfish together without an issue unless it's a fancy tailed goldfish and you have a male betta. Bettas are usually kept by themselves because the males are aggressive towards each other and often, they will mistaken other goldfish with a fancy flowing tail for another male betta.

Although it's true that bettas are from warm climates, many bettas are kept at room temperature in small bowls without a heater, so they are adaptable. Since most goldfish can live in 65 to 75 degrees F water, if you are using a heater, I would set your tank to 75 degrees so the betta is happier.

Depending on what type of goldfish you have, it can get very big, so you may want to look into that before deciding the size of your tank. The betta should be fine in a larger tank -- they were originally found in rice patties in Thailand. I have a betta in my 65 gallon tank.