I absolutely would not keep pearlscales in a pond, personally. They are too slow to defend themselves against predators, and need a little bit warmer temps than commons, comets or shubs.
Pearlscales can reach softball size.
I have three - two hamanishiki and a tiku. I got them at around an inch big, they have about doubled in size in the last month. As already stated, they can be prone to buoyancy issues so I feed only homemeade gel food.
As far as tank size - saying that you need 15 gals per goldfish and 10 gals per additional can be misleading. In my opinion, no goldfish should be kept in less than a 55 gallon tank. Second, no goldfish should be kept in a group of less than three. I also feel that 10 gallons per goldfish is adequate, AS LONG AS your tank is 55 gallons or larger and your maintenance is up to par - that means 2-3X filtration, and 1-2 times weekly water changes. For those newer to keeping goldfish, barebottom tanks are the way to go.
Kristina