Again, it depends on the goldfish. The fancier varieties typically do better with a consistent temperature in the mid 70s -- we're talking pearlscales, ranchu, celestials.... I've kept fantails and telescopes in a pond, one of mine has overwintered in a pond its entire life, ryukin also are pretty hardy and can be kept in a pond or unheated tank.
If you wish to breed them, any goldfish needs a 'winter'.
The fancies (egg shaped, split tailed) can typically be stocked at 20 gallons for one and 10 for any fish thereafter, provided nitrates can be maintained under 20 and with the knowledge that surface area/oxygen exchange is crucial kept well in mind. (a 30 long is better for goldies than a 29, a 90 solely increases gallonage w/o increasing the surface area of a 75....) The single tailed, torpedo shaped ones need more space to do their zoomies. :goldfish: