turtles eat fish and if they cant fit them in there mouth im sure they will annoy them to death by pecking and biting all of there scales off, some turtles are just plain scared of all fish. And most turtles leave the pond and end up in a parking lot. Snails are allways fun
you can allways try blue or gold gourami and a betta. Assorted male and female fancy guppies with real or fake floating plants should be nice over the summer make sure a crayfish has access to air , for example a rock island in the middle of the pond, the crayfish might end up trying to make it to a parking lot too.. ive heard of all tropical fish being bred outside in buckets or vacationing in the pond. But you must still make sure your fish have enough room. Some dudes quote---> General rule of thumb is 200-250 gallons per Koi and 100-150 gallons per Goldfish with proper filtration and partial water changes. If you have sufficient filtration, more can be kept but you are always in danger of something going wrong that will be harmful to an over populated pond......
Try Endlers live bearers they dont care how many of them are in a pond they will be happy to not be feeders