The Koi you already have will grow quickly. As your fish mass increases, so must your filtration and pond keeping skills. Adding more Koi only exacerbates the situation. Assume that the sore on the side of your Koi will just go away. If a sore just appears, its a sure sign of water quality problems.
Dirty filters are a breeding ground for parasites and bacteria.
Decaying material also gives off poisonous gasses. Keep your filters clean. Come home hours later to chlorinated Koi. Never just turn on the hose and leave. At least use a timer, but be very careful about not putting too much faith in a timer.