I would agree on the betta and a small heater. they aren't pricey to buy and run on a tank that small.
White clouds are an okay choice but the tank really won't allow them to flourish liek a minimum of a 10 gallon or larger would. White clouds flourish best in large groups whith fast flowing cold water.
Also how cold is this tank? Just because you don't have a heater doesn't actuallymean it is cold water. Cold is really under 70F. 74-78F is generally fine for most tropical species. A heater is just useful for maintainign a stable temp is most cases.
As for how long could a goldfish last? Well quite a long time, but they will end up stunted and will remain small for the rest of their lives which will be shortened. I have seen goldfish live for a few years in small tanks or bowls. But they should be able to live 20 plus years. And once stunting occurs there is usually nothing that can reverse the process.
