Unpopular opinion but I have a Betta and 2 doubletail goldfish in the same tank. I dont heat my tanks through summer and winter here doesn't get very cold but I have a heater for winter just in case. The house is climate controlled because for multiple tanks it's more convenient to heat the room than several individual tanks. You need a tank big enough for the goldfish which will grow quite large. If the Betta has lots of plants to hide in he'll be ok. Tbh it's not the best matchup but my goldfish were rescued from a pond full of large koi someone had dumped them and the pond was warm to the touch 30 degrees Celsius all summer so being in a 26-27 degree tank isn't gonna hurt them. The Betta has grown huge bigger than any Betta I've ever owned and the goldfish don't worry him. They just like to rest a lot compared to goldfish who are very active, bettas like to bask near the surface sitting in a leaf or W/E.

Tbh depends how big the tank is a tank big enough for a goldfish is going to be bigger than what you bought. Goldfish need a lot of space relatively to other fish.