All turtles will from time to time eat fish while some turtles eat a lot of fish. If a turtle is hungry, and there's not too much to eat, it may eat the fins off of larger fish. They usually don't die from that but could in a very small pond. What turtles you might get really depends on what turtles you can find. There are many species but most that are sold are red-eared sliders. They will eat some small fish and plants. Painted turtles tend a little more towards eating animals but mostly smaller ones like insects and worms. It's rare to find other species at a store. If you contact your local turtle or reptile club, they may have breeders in the area or turtles needing adoption. Mud turtles stay smaller and mostly eat insects and worms. They are just brown though and hard to find since they are smaller. Turtles under 4" are illegal no matter their age but you'll find them for sale anyway. The little ones are less apt to run away if you don't fence the pond which you should. Little ones are also more apt to be taken by predators like herons and raccoons.
If you want a turtle in your pond, you will have to accept that it will sometimes eat some of the smaller fish and some plants.
My page on turtle ponds -
http://www.fishpondinfo.com/turtles/turpond.htm