First of all your tank is way too small. The minimum tank for one baby koi is 100 liters and this is for the first year you have it. Goldfish need about the same amount of water for one fish but they grow slower. If you start with a 100 liter tank for one fish you add 50 liters for each new fish (this is for both koi and goldfish).
When you get the medication against ich try to get one that is only treating ich or you will get problems if it kills the good bacteria in your tank and filter.
If the koi have a wound you need to add some aquarium salt and the dosage to use is 0,3 %; this is a safe concentration for long term use. The salt will neutralize the effect of nitrite in the water and stimulate the healing of your fish by increasing the blood flow in the skin of your fish.
For now, I think the only thing you need to do is to keep an eye on the injury on your koi. If you see it getting red or it becomes infected by fungi, you need to start treating it.
The common treatment for small injuries in the skin of koi is potassium permanganate dissolved in water; you can buy this in any pharmacy. It is a liquid and you apply this by a q-tip directly on the wound of your fish twice a day for a week. And try not to spill any, it makes everything purple.
Remember not to buy the powder but the premixed low concentration potassium permanganate, the type used to treat wounds on pets and people and now fish.
Try to keep the temperature around 25 °C = 77 °F; it is best to cut back on feeding for the time you treat against the ich.
Fish in the carp family are seen as dirty, they produce a lot of waste so you need good filtration of the water.
And please get a larger tank for your fish.
