Well Anja, I hate to break it to you, but there are going to be a lot of replies to this post due to your stocking, and very few towards ich. It is hard to realize, but there is a lot wrong with the fish you have chosen in terms of adult fish size, temperature compatibility and aggression issues. Disease will be a long term problem in this tank until the stocking issues have been fixed, but the ich is more immediately important. I have some more personal advice for you first though ok?
Take a deep breath, and steel yourself. Yes, you have made mistakes, but here you are trying to fix them. Everyone here will give similar advice, all usually true and founded, but sometimes it will come out mean and harsh. Don't give up. Keep at it and don't let things get to you. Read the posts, do independent googling, and ask any questions you have. If you truly want to save your fish it will take some time and work. Keep at it, and don't give up.
Now for the stocking. First off, koi and shebunkin tend to get very large- 12 inches long or so if I remember correctly. These alone would be a good stock for your tank. But then you have gouramis, tetras and a bala shark- these fish require warmer temperatures and quite a bit of space. Bala sharks also get very very large when full grown if I, again, remember correctly.
If you would like to keep all of your fish, then I suggest getting another tank of the same size to move your two goldfish into. The other fish should be fine in the original tank, minus the bala sharks. I think the only real choice if you want to give them a good and full life, is to return them or get a really big tank. I am sorry for this.
This advice is not overly comprehensive or full but it is a starting place for you to do some research. Now for the disease.
Raise the temperature way up- like 84 degrees or so if I remember. This is a good way to help stop the spread of it. Do a lot of googling and searching on this forum, as there are a million and one ways to cure it (I have never had to cure it so I couldn't even tell you how I do it) but find one that seems legitimate and try it.
I hope your tank pulls through and so do you. As I said, take a deep breath and just be ready to work through this. Be patient and try your best, and your tank should pull through.