They should get along fine but catnapper is right about needing extra tank space later. Also, make sure your betta gets enough to eat, goldfish are hogs! Sometimes they gobbled things up faster than my betta could get to the food because he was always looking and stalking the food before he'd strike so I set up a little plastic "cage" part I could block off with a removable side to help make sure he ate enough. But if you see that he is being able to get to the food fast enough before they eat it all it should be fine.