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You really should get drop tests for your water, ammonia, nitrite, and nitrate. If the fish haven't grown in a year, its usually due to bad water quality. Young goldfish should grow 3 inches per year. The koi will not grow to the size of the tank. The reason fish get stunted is because of poor water quality in an overstocked tank. That poor water quality eventually kills the fish.
Your fish are really pretty!
I know it doesn't "seem" like you have alot of fish since there's plenty of room for them but bioload is a delicate balance. I think I read somewhere that freshwater fish require a minimum of 1 gallon of water per 1 inch of fish. (When figuring it out rember to use the average adult-fish length.)
Some will quibble with this rule -- arguing for more gallons of water per inch of fish and I can definitely see the safety in that. (More water = bigger buffer to toxins, etc) That said, I think alot of it is up to the person and how dedicated they are on water changes - not to mention the maturity of the tank.
And here's a shameless nudge (DO IT! LOL) for your future plans of a saltwater tank! :D I absolutely love my little reef.
Your fish look really pretty! I love fish. I have a koi and he stays at my dad's house. He got too big for my 30 gallon tank. My dad has a 55 gallon and he has gotten HUGE! His name is Cheeko and he is as big as some fish that you'd catch in the river! He's white with orange splotches. I really like him, and it's fun to watch him grow.