Sounds like you have the right idea about doing things to ease the spring transition for your fish. I am in a much colder area than you too, but I would not just automatically use medications like a shotgun to prevent problems. I belive a better way to go may be to buy a water test kit and monitor the water quality parameters. Test for amonia and nitrites as your bio filter restarts. If the water quality is good the fishes imune system will have an easier time coping with problems. Partial water water changes are great for keeping water quality parameters up. Salt works well for thickening the slime coat on your fish and it helps protect them when the water gets a little funky, but keep in mind that it can effect your plants. As far as your fish being ready to spawn this year I would doubt it. 12 inches is pretty small for koi to breed. Females are usually at least 3 years old before they are ready in my area and at that age they are usually far larger than 12 inches. I have heard of some fish that were precoious or stunted that spawned at that size though. Several years ago I had a 3 1/2 year old female that was only 12 inches long spawn, but the eggs did not hatch at all and there were not very many eggs. I kept that fish for another 2 years, but it never did spawn again and never did grow much more.