Stable ginrin? I did not know there was such a thing as
unstable ginrin. Every ginrin koi that I recall having stayed about the same as far as I noticed. I have heard it said that ginrin koi have more shine when they are very young. I have not observed this myself.
I don't know much about bonsai koi. You might check some of the information on this thread:
https://www.koiphen.com/forums/showt...354-Bonsai-Koi
Seems like it would be especially challenging to grow koi large in a cold climate. Two keys are plenty of food and lots of water. Best chances are with female solid colored koi known for and bred for growing large such as chagoi, soragoi, karashi, or yamabuki ogon.