I have 2 red-eared sliders, who are both several years old now. The female is about 10-11 inches long, and the male is about 8 inches long. If it wasn't for the fact that they crap soooo much, and piranhas are such messy eaters, I wouldn't think twice about putting them in a tank together. I don't have enough filter power to handle that, nor do I have the time to clean the tank every other day. But, if I did do this, I would worry about the safety of the piranhas (especially if they're under 3-4 in.). My red-eared sliders are just downright vicious.
Everything I put in there get devoured in noooo time. They are fast, smart, strong as hell, and have a bite you would not believe (I can show you my arm where the female got me). I have put african clawed frogs in with them, frogs I found out by my creek, goldfish, feeder fish, minnows, tetras, and even a small snake (they killed it but didn't eat it). Everything gets ambushed and eaten (or chased for long periods of time). The piranhas would learn their lesson real quick if I put them in with my turtles. My female would endlessly chase them around the tank. I know for a fact my turtles don't have salmonella, because my Mom makes me get them checked every once in a while.
If I had a small pond in my backyard with ample room for them to share, I would put them in there, because I would have an area that's about 3-4 inches deep that my turtles would hang around in all the time, plus they would bask out in the sun all day regardless. The pirahnas would stick to the deep parts. Of course, I know my sliders wouldn't last 15 minutes if it was a large shoal of pirahnas out in the wild.