You might be able to get them to survive but I wouldn't spend too much if all you want is an occasional sighting.I would wait to add koi. Conditioning is not going to be easy on them at this time of year in zone 5. What is your anticipated weather in zone 7?

Sorry for your fish losses. I know how devistating that is.
If it was me, I would wait till spring and get the fish from a local pond/fish place. Cheap fish of unknown health, from a indoor warm enviroment, going into a cold pond may just give you a pond of dead fish come spring.

Other things to concider are; If you have a heron, he will be back for more fish this year. If he comes back and finds no fish, he will take your pond off his list and look for a new dinner table. Also if you add fish now you likely wont see them till spring anyway.

Mike

Dont most fish come from Florida? I wouldn't.