From my personal experience and judging from the responses I would say it totally depends on the specific turtle. Some may co-exist just fine with gold fish, while others will bite at them every chance it gets.
I have two separate tanks of turtles, each with fish in them. My large tank has a RES, painted, and map with a very large oscar with them. Even though the RES is a good amount smaller than the oscar, I still catch it biting at the oscars tail.
On the other hand, I have three gold fish in my other tank with a map and painted. These turtles have never shown any aggression towards the goldfish even though they are about the same size.
So, I would say give it a shot and monitor things for a while. You'll know fairly soon how your RES is going to respond. Good luck.
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Mike
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2 Painted
1 Musk
1 Common Snapper
1 Cooter
1 Map
1 RES
1 Corn Snake
1.1 Water Dragons
2 African Frogs
1 Toad
1.0 Degu
1.2 Chihuahuas
0.1 Cat