Ya. The 10+10 rule is really only good for larger tanks (40 gal and up); and even then it can result in overstocked or just on the line tanks. There is no hard and fast rule for goldfish in smaller tanks because people (who understand the needs of these fish) want to keep lots and therefore keep goldfish in large tanks.
A ryukin would be a good choice for this tank. While I have seen pictures of massive ryukin, those are typically show fish. I don't think the normal fish you get from a pet store will get that big. My ryukin is the tiny one of the tank. He's only about 7 inches long.
What filter were you planning to buy? As I saw recently, filter quality does make a difference with goldfish. I was helping someone through a fishless cycle, but their cycle stalled. We determined it was because the filter (just your basic cartridge-type) didn't have enough space for enough BB to support goldfish. This poor girl had to go buy a canister in order to get her tank to cycle.
I thought about this a lot last night, and I think it could work. The tank is as long as a 40 gal breeder which has been established as a good home for two goldfish. You will need to keep a close watch on water quality as it can change rapidly with goldfish and cram the filter with bio-media. And you will be limited to only one goldfish. Impulse buys are okay with fish like betta that don't grow as large, but with goldfish it can spell dooooooom for your tank. Basically, the take-home message is it can work but with caution.