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Fancies tend to fair the best in 70-75f range. Excess heat can be deadly and shorten their lifespans in general due to increased metabolism.
Cooler temps are tolerated (to a point) but they also have the opposite effect of heat on metabolism and digestion- cold slows it down. Not a big deal for slim comets or commons, but a fancy's round, compacted body and organ system can be affected adversely by slow digestion resulting in buoyancy issues and/or possible intestinal infections. Cool or cold temps slows activity in general. Some fancies can not tolerate winter level cold temps (50f and below) to any degree nor are they able to tolerate rapid temp fluctuations. Mainly an issue with fish kept outside. Hope this helps a bit! Feel free to ask any questions!
66 is fine. Optimal temp for good growth and for breeding is around 73F, although they will breed at 65F. Some breeders raise the temperature to 80F to stimulate spawning.
Goldfish are classified as warm water finfish, so they are warm water fish that tolerate lower temperatures.