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I run a freewheel on a fixed thread
If the freewheel has the same chainline as a fixed sprocket, it can't put more stress on the hub thread. Of course it looks frightening having all that unengaged thread on the freewheel body, but it's not doing any harm. Remember, in the olden days our top sprocket was hanging off the threaded part of the hub by about an inch with no support, and we didn't have any trouble.
gbj_tester-
Unless we had an 8 speed freewheel. So many cheap MTBs with 8 speed freewheels used to come in with snapped axles. I replaced many, or just replaced the wheel. Despite me pointing out they'd be far better off spending a little bit more on a freehub wheel and cassette.
I used to think they stopped making them at 8 speed but you can get 9 and 10 speed freewheels too.
tallsam
Yeah as others have said you'll be fine, I run a freewheel on a fixed thread too no issues so far!