I have a Kreffts turtle. They are a pretty common turtle and I just bought mine from the local pet shop. I had him in with another one the same size but they kept beating one another up.
If you are going to keep fish with turtles you need to grow them up together preferably and they have to be a good size otherwise they will eat them. A few of my cod and barras have chunks out of there tails. Turtles will eat anything, it doesn't matter if it's too big to fit in their mouths. Mine rips fish to shreads with his claws once he gets a hold of one. I usually buy him feeder goldfish as they are a little slower for him to catch. I also breed platties in a bath tub in my backyard for food as well as they breed like crazy!
Also you really can't have plants in with turtles. They do need them in their diet but don't expect them to leave your $30+ plants alone. I had some really nice ones in my tank with mine and he destroyed them all in a matter of days. I just have elodi now, grow it in the bath tub and put a few pieces in ever now and then. Other than that I have just a few fake plants. It doesn't ever matter if they don't like the taste of them. They will still rip anything to shreads or dig it up. You name it, they will destroy it!
I have introduced my snakes to my turtle. Only so they can see each other through the glass of the aquarium. The snake is fine but the turtle tries to jump out of the water to bite the snake. If It was me I wouldn't have the two anywhere near each other. It wouldn't take much for the turtle to grab the snake and claw at it. You would also need a baskin dock for the turtle as well, the water heated, a baskin light and uv light. If anyone reading this has turtles don't feed them turtle dinners or turtle sticks. Feed them hikari gold cichlid pellets.
Here's some photos of my snakes and turtles.
The last one is of him on his basking dock. Most people I've talked to say they don't often see there's basking. I put food on mine so my turtle has to go up on it. They will do anything for food! I've been doing it for a while now and he always goes up on the dock now to check if there is food haha! They are nice to own and mine is really tame, he doesn't mind being handled at all. They are extremely messy! I have one of the biggest canister filters I think you can buy and he still fills it up pretty quick.