I have had a loggerhead musk turtle for the last 3 years. I have moved him from tank to tank and he has been fine with all tankmates. There is no way that the jack dempseys will hurt him. I would bet money on it. My musk has been a little food aggressive at times, but has never done any serious harm to any other tank mates. Also, musk turtles are not very good swimmers. They usually will walk on the bottom of the tank and occasionally come up for air.
I have a basking spot for my turtle, but I have never once seen him actually using it. I have only seen him out of the water about 3 times since I have had him. Overall, they do very well with fish and I would not be afraid to add him to the tank. The only possible harm that could come would be if one of the fish tried to take food from it. In that case, the turtle may bite the fish out of food aggression. I would put the turtle in the tank and monitor it. The jack dempseys may go after him, but they will not be able to do any serious damage.