I used to love getting goldfish from the county fair. I had one named Freddy, who died after 5 days, which was basically a miracle considering the shape that it was in. But anyways, back to your problem. Goldfish require a lot of space in their tank because of their bioload, so consider the room that is available in your tank. For the best end-results, its advisable to have 20-30 gallons per large fish. With smaller fish, you can get away with 10-20 gallons per fish. Regarding the temperature, it sounds like you are well aware of the temperature requirements for goldfish. All goldfish are 'coldwater fish' and do best in temperatures of 68-75. Though they can survive in temperatures from freezing to 80-some degrees, it is not recommended to keep goldfish permanently in these temperatures. Finally, because you do not know where exactly the goldfish came from, by introducing the fish you would put the other inhabitants of the tank at risk for disease.
I would not recommend releasing your goldfish in your lake. I don't think that the fish would do any damage because it would probably get eaten by something bigger than it within days.
Hmmm. You are in a tough situation. I don't really know what should be done with the fish sorry.