Yeah its fine. I had a fixed/fixed hub from when i played fixed, and when i went freewheel just pulled of the cogs and on go the freewheels.
They thread on to the bigger portion of thread, and the lockring thread sits there unused. There's a little bit less thread engagement (obviously), but there is sufficient contact.
Never had a problem, never heard of anyone having any.











I do it. It's all good.