I used have two musk turtles with a red tail shark. The turtles did nip his back fin once or twice. That was as far as it went though. Ive seen a few videos on youtube with musk and fish together. But id be worried that the betta has large fins which get shredded. So i doubt it would be a good idea.
Mine died couple of years ago. Had them for ages they were great. I still have there tank in the hope i will get some again some day