Zydia,
I live in Omaha, Nebraska, zone 5. We had tons of baby fish last summer and I left them outside last winter, they weren't very big at the time either. I didn't lose any of them. Just be sure to keep a hole in the pond whenever it ices over. We used either a pond de-icer or a bubbler, it depended on how cold it was out. If it was bitter cold we used the de-icer, otherwise the bubbler apparently did the trick just fine because all the fish lived.
Hope this helps.
Cath