Koi will do great in there. From what I have seen is the RTC do fine during mild winters if they don't get ich. Then they die during a normal colder winter. Pond size doesn't seem to matter. I have seen the same struggles in 100'x50'x5' ponds. Some other good fish are hifin banded loaches, channel catfish, iridescent sharks, gymnogeos, red ciebals, jewel cichlids, swordtails, tinfoil barbs, rosy barbs, pacu, I have even heard people having more luck with TSN, but the sample size was much smaller and 9a near the 9b line.