Goldfish will be fine as long as it doesn’t freeze solid. As for the shark eating goldfish, one or two might be no problem, but they contain a compound which will inhibit the sharks b vitamin absorption if it eats too many of them on a regular basis. You might set up a different tote or large aquarium to quarantine feeder fish such as minnows, shad, and when the sharks are bigger, other edible fish that don’t have the spines on their fins, like sunfish do. Menewhile, blood worms, mosquitoe larva, brine shrimp are good protein sources that are commonly fed to small carnivorous fish, and can be purchased in frozen cubes.
If you find that the water temps do drop too much despite heaters, to keep the sharks out in winter, consider creating an indoor pond if you have a suitable room for it. Concrete floor. The indoor pond can be created by making a strong frame, with a liner in it, and it could make a very pretty water feature with plants, perhaps a bog filter, if you add a grow light you can grow your own herbs and fresh vegetables year round. YouTube has plenty of videos, for how to and ideas.