Hi, I live in Idaho (cold northern winters) and last summer I rescued a bunch of bass and bluegill from a small lake or large pond since it was going to be drained. I have about 12 fish living in a rubbermaid type 800 gal tank that is about 28" deep or so and maybe 7" across. I added a 50 gal biofilter I bought cheap off of craigslist and put in a skimmer and pump to circulate the water. In the winter, I had a tank heater running when it got really cold and I built a wood frame above the pond and covered it with clear plastic. I never had a problem with freezing and the fish made it nicely all winter, though I never saw them after about October.
The fish are all still alive and are about to be transferred to a pond of about 160,000 gallons. I might mention that at least 3 of the fish are 17-20" smallmouth bass, a half dozen smaller smallmouth and a few bluegill. While it seems crowded, all of them have done quite well and two females even appeared to have eggs or nests? they were guarding this spring.
All that to say, it can be done and with quite large fish. I have lots of green growth, some things to hide under and behind and feed them about every two weeks when the water is above 60 or so and of course I test the water. They won't take pellets or other fish food, so I feed them worms from my yard and man, what a hoot to have a 20" bass take a worm right out of your hand. Take comfort in this...I had about 3 days to prepare to take the fish and knew nothing (0) about fish keeping and mine are doing great. I'm a lot more worried about this large pond I'm setting up.