The above should work, can you check for orphan (I think that's the one) cells when you switch on overset? If you have too many of those things tend to go wrong. Then, if you work out the terminal velocity of the particle, how many time steps will it take to cross one cell of your domain? Ie are you resolving time correctly?
The 6DOF example in Fluent's Help ought to cover what you're doing very nicely. But I suspect it's all laminar flow. Just check the domain scale to make sure you've not set something as (near enough) zero mass.
Moving the fluid will allow forces to be calculated, and you can run transient to see the shedding effects. Torque will be an interesting one, and I'm not sure how large a domain you'll need to allow the solution to develop to see if you get a tumbling effect: sphere/circles are notoriously hard to model.
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