Anthony;
I keep turtles and have since I built the pond. Actually, I had them from almost quarter-size and one of my motivations was to get them a larger environment, aka the pond. They grew up together (two females) and rarely showed any aggression but were outgrowing their 50 gallon aquarium. I too was worried when I eventually put them in the pond and did my research. Here's the skinny:
you have to have some sort of fencing if you plan on keeping them there. Since my pond design already included a porch/enclosure with window screening--to keep out the mosquitoes--the turtle's ability to wander was nipped in the bud. I had read that turtles WILL wander, esp in the spring when mating calls.
And you need the pond to have some sort of 'muck' at the bottom if you're going to overwinter, plus the pond should be at least 3' deep, imo, to properly provide for turtles 'in the wild'. My biggest fear was that they'd go after my fish and so for a year, I had a divider/screen that separated my pond into halves; one side for the 17 gf and one side for the two female painted turtles.
Eventually, the fish had babies and some slipped past the screen and grew into actual sustainable fish ON the turtle side. I doubt I'd have so many fish now (~90) if the adult fish had been able to get at the fry. I have no idea if/how many fry the turtles ate but didn't see much 'chasing' until the fish got larger. And the turtles did try and catch them but I never saw even one get taken, though a few tails looked like they'd been bitten. All that said, I knew some day I'd have to lift the screen and see what would happen. When one of the two females began to harass the other, I knew it was time to relocate that one. When the remaining female found a way to get to the 'other side', it was time too to lift the screen and observe the interaction. Didn't seem that there was much more abuse/chasing than before so everything seemed fine. Then the next spring came. That's when the remaining female was daily outside the pond and looking avidly for a way to escape, no doubt feeling the mating call and the nearness of the 'large pond down the road' where other turtles were located. Eventually, when I THOUGHT one of my largest and favorite gf was a turtle victim, and since the female was so anxiously trying to 'get out', I figured it was time to relocate her as well. Found out a few days later that I erred in that my fav gf was just fine but it was too late now to fetch the female turtle back. So I then went and got two more small ones to raise up (again), but this time in my pond.
So to recap, you WILL need secure fencing--I heard it should be vertical, small opening mesh, about 18" high at least. Some reported their turtles climbing so angling the fencing inward at the top would help as then, gravity would assist you. To over winter, they need something to burrow into while they brumate (hibernate), and I'd feed them turtle pellets and the like to keep them from looking too closely at your fish, though that's always a risk you take with turtles. Since mine were raised sans fish for almost 6 years, I figure they were used to the pellets and didn't need to 'chase' after their food, so maybe I was lucky. Still, be aware that putting turtles with fish means you might lose a fish someday. I think having some underwater pond plants helps this too as aquatic turtles are mainly herbivores when they get older, preferring more protein when they're still growing.
Anyway, that's my experience and I do love to have the mix of turtles and fish (and frogs and snails and dragonflies!) to go along with the plants and the whole pond experience.
Michael