There is plenty of flex in WSF
Make sure to keep the wall thickness > 0.7mm.
Preferably double in the "working/bending" parts.
For a post-print snap joint I would not use any extra clearance.
In my experience negative spaces (socket) tend to be every so slightly smaller by 0.1-0.2mm. And positive spaces (ball) 0.1-0.2 mm bigger.
So chances are it will work as it should, if you make the ball the same size as the socket.
If you add some extra material, I would not go as high as 1 mm, that would be very tight I think. Stay under 0.5 mm perhaps.
But a lot depends on the actual design (and probably print orientation, over which we have no real control), so best is probably to test-print a few joints. Preferably with multiple orientations in one file, to make sure it works regardless of print direction.
Cheers,
Stijn