I'm sorry but your pond is not big enough to keep Koi. Koi are supposed to grow to nearly 3' long so a 6' pond is much too small. You should trade the koi or rehome them.
Based on your dimensions, your pond is around 800G US so you should stick to comets and/or shubunkins which usually grow to around 12" but sometimes grow larger.
Or if you are certain about keeping them indoors during the winters, you could go with fancy goldfish but it would be best not to mix the fancy and long-bodied as the fancies can't compete for food as well.
Try to give them 100G each but if you have good filtration and airation, you could go with 50G per goldfish. Remember they will probably breed and you could find yourself severely overstocked by next spring.