I don't think a Betta would be a good choice for your goldfish Bettas like warm water .I had several bait goldfish given to me at the other house I put them in the stock tank , it had a heater to keep it from freezing . They got huge and reproduced . How about another gold fish ? Can put a Betta in a community tank but the other fish pick on them nipping fins .

Poor Clyde stayed up all night surfing lol . When you get them at a Pet Shop they are mature the life span is not that long for them . Thanks for looking at Bubba . Gin