The koi you already have will grow quickly, As you fish mass increases, so must your filtration and pond-keeping skills. Adding more koi only exacerbates the situation if you're not equipped to handle it. Assume that the sore on the side of your koi will just go away. Bit if a sore just appears, it's a sure sign of water quality problems. Put off cleaning your filters for just a little while longer.
Dirty filters are a breeding ground for parasites and bacteria.
Decaying material also gives off poisonous gases. Keep your filters clean. Drop the hose into the pool to top it off and run to the store for a quick errand. Come home hours later to chlorinated koi. Never just turn on the hose and leave it.