I would try it, but I would also have a spare tank up and ready for when It went horribly wrong! Its just something your going to have to risk and try out. My money is on the fish getting nipped at.
Well I think I'll just tell them at the lfs I'm going to try it and then I'll return it and try to figure something else out if it doesn't work. They're pretty cool at my lfs so they should be fine with that.
I'd say that it depends on both the fish and turtles temperment, and if you will have proper filtration. If you do go for it make sure that you post pics!