If the fish hang around near the surface and skimming the surface film, then they are likely Gambusia. A close photo of a few would be helpful for a positive identification. Gambusia are easy to raise indoors. Instead of trying to get them to reproduce, focus on keeping them healthy (good filtration) and getting them to adult sizes so when spring comess they are ready to start reproducing as soon as the water gets warm enough. This then should produce a big crop of them next summer if your fish don't eat them as fast as they are hatched.

aka Pond Doctor & Dr. Perca Read Pond Boss Magazine -
America's Journal of Pond Management