Yeh I've seen a few tanks where fish and turtles live happily together!
Firstly... Avoid amphibians, they can be toxic. And shrimp and crabs could be a tasty treat for your turtle
if you have lots of hiding spots then ghost shrimp could work?
Avoid Plecos because they tend to be lazy and might get harassed by the turtle. Not to mention, they create a lot of waste too!
Snails are ok but they might get eaten. There's mixed views on snails and I dont want to comment just in case.
Any small fish will be a tasty snack for a turtle.... If it can catch it
try to get fast swimming fish and preferably keep them in shoals so that one fish doesn't get constantly chased. The stress spreads across the group then.
The better idea is to get something slightly smaller in size to your turtle. I'd probably go with 4 inches long. Too big and you might spook your turtle. Too small and it might get chased. The majority of tanks if seen have a lone cichlid fish with them. They are aggressive enough to stand up to the turtle but not too much that they attack it.
Just in case you haven't thought about it.... What about another turtle?
you can never have too many turtles
Hope it helps!