My turtle has lived quite peacefully with some mollies, platies and a pleco for a long while now. I thought then he was tolerant of fish and added some female bettas - turns out he's just a picky eater. He loved the bettas. There are a lot of folks that have stories of a surviving fish or a type of fish that the turtle wont eat, but it seems to be idiosyncratic with each turtle. Just try it out. For what they are worth here are my suggestions -
*Dont put barbs, sharks, or other fish with sharp or hard fins and tails in with your turtle.
*Dont put very aggressive fish in the tank with your turtle. You dont want your little guy picked on.
*Dont put goldfish in the tank as they are not recommended for food.
*Dont put expensive fish in with your turtle. You will probably lose them and you dont want to be out $29.99 for that beautiful discus you found.
*If you are lucky and have a picky turtle, he still might prefer fish that are "fancy", long fins, bright colors. I would assume tetras, fancy guppies, bettas and other flashy fish would go fast (but who knows really). I have orange mollies with long fins and platies and they have been fine so far.
*If you have kids remind them that turtles eat fish - its a cruel world. Mine were very upset with Tuck when he ate the betta. It took a while for them to make peace

I have to admit I was a bit upset too. Bettas are beautiful.
We love our fish and Tuck. The fish make the tank seem more like a natural river or pond environment. I am glad they are getting along so far - hope it lasts. The fact is though you may have to let him eat a couple to figure out what your turtles temperament is. If you dont want him eating any fish then I wouldnt test it out.