whats great about koi is that they can adapt fairly well with different water cond't . a 6.5 to 8.0 ph levels is possibble for killerskoi to handle, water hardness they deal well with also. So thats a plus for u another plus is in ur neck of the woods the weather is BEAUTIFUL!!!!! so all u really need to do is find what fish it is that u want to go with and reinacted the water conditions give or take certain extremes. people who atempted it haved nothing but succsex the only precautions that u might need to take is temp. a pond heater will fix that problem quick and just watch and keep the temp. stable durring those few brisk days&nights that u might have( remeber anything under 65 is freezing for tropic fish!!!! And thats really the biggest obstacle u will have the fish that most people use for this type of setup are (red epresses)(Venustus haplochromis)(electric blues)(chichlids) and more one of the coolest and a wild fish to go with are OSCARS they dont even mess with the KILLERKOIs as long as the koi are not tiny,but that can go for koi to. yes koi are no aggressive but they are fish and if it fits in the piehole its a goner and since since koi are gentlegiants theyand the oscars dont grow as big as koi the get along great a client of mine did this and it was very cool if u need anymore info on this subject just email me it or post it and i'll help u further dude P.S.... KILLERKOI RULEZ
Here are a couple shots, I have known several people who kept any or all of these with koi in ponds. There are several sunfish, this is the bluegill, you can catch several types locally, same is true of bullheads.