Older cassettes used the small cog to hold the rest on
"Thread on smallest cog and secure clockwise with sprocket chain whip tool."
Teardown? It just needs 2 chain-whips,
One holds a cog on the splined center portion,
pulling in the opposite direction, [CW] from the one unscrewing [CCW]
the smallest cog.
In practical terms , Building a single speed wheel is better ,
you won't gain the improved wheel strength ,
leaving it dished, imbalanced spoke tensions,
to make room for 6 speeds.