Originally Posted by LebronTheBetta
I don't think they can breed...
All breeds of goldfish can breed with each other. Goldfish are egg-scatters, so even though they may look different the males can still scatter their sperm on the females eggs. But a male common might beat up a female fancy pretty badly. Goldfish get rough during spawning.
Two comets in a 50 gallon pond would probably be stunted. In a pond that small regular water changes should be done. Without those, there is probably some stunting due to an excess of hormones and nitrate.
It appears there can be single-tail fancy goldfish. I asked my goldfish breeder friends and they told me about a very rare breed from Japan called Sabao (or sometimes tamasaba). They basically appear to be single-tail ryukin goldfish.