Why's everyone hating on the fancy goldfish???
Ryukins, orandas, etc are pretty cool dudes, but they are messy. You didn't mention if you got a heater or not, but with these guys don't plug in the heater, cooler water is better. I'd just avoid the plants, they will dig them up when they get bigger and anything with soft leaves will get munched on. One or two will be fine in a 20, probably will have to move them in a larger tank in three or four years however.
Just keep up on the water changes and they will make it through the cycle just fine assuming you just put one or two in there. Might want to consider bumping up the filter size though. Makes life easier.