We have a pond in my backyard, where we have close to 50-60 medium-size goldfish. The goldfish have grown quite a bit since they've been in the pond, the youngest being about 3 inches, the oldest 4 inches. Since a turtle wouldn't be able to fit the goldfish in their mouth, would my goldfish be safe from the turtle if I placed it in our pond?
I would say that the goldfish would not be safe depending on how big the turtle is. I used to have a turtle, and every once in a while we would get some goldfish to feed him as a treat. He would usually swim around and try to nip at them. He literally would go and eat them bit by bit. It was pretty messed up. but it took him about half an hour to finish 4 or 5 medium sized goldfish.
Now, your goldfish sound a lot bigger than the bait fish I was feeding my turtle. I am not sure what type of turtle you have, but it doesn't seem like much of a stretch to say that he could easily start nipping on fins and eventually start making kills.
One thing to keep in mind is, if you are thinking of keeping a smaller turtle, like the kind that are the size of someones fist, he might be scared off by the large swarm of goldfish. Goldfish in my experience tend to swim more a less in groups. He may be overwhelmed and just leave the fish alone.