I can't speak for it fitting the WF600 you linked, it definately fits the Ultrafire WF500 (available in various different configurations, including that one).
On a single 18650 I wouldn't be confident an 18650 could deliver the power needed for that unless its horribly under driven, which it doesn't appear to be judging from the beamshots in
this thread.
A better solution while keeping it compact is probably two 18500s (Maybe need IMR depending on the current draw and how long you want them to last) in a WF500 without the extender.
