Erica--My gut feeling, rooted in experience from many years of overhearing
guides misguiding visitors, is that they don't like to tell people "I don't
know," and will make something up on the spot rather than admit to lack of
knowledge.  I've heard some truly remarkable "FAWBITS" (fictional accounts
without basis in fact) from guides who felt themselves to be on the spot
without the necessary information.  Best to avoid that situation when
possible, so while I understand your position and need to fill docent slots,
I wonder if it might not be possible to comfortably straddle both sides.  

I agree that working as a docent is a great way for newcomers to learn about
the community, so for each room, gallery, or exhibition you might compile a
notebook with comprehensive information on each object, more than what might
be communicated by a label.  Keep these behind a door or in a drawer where it
is accessible to the guides.  At least the information will be at hand and
easy to look up so that the visitor might go away with an on the spot answer
to their question, while the docent has avoided "loss of face."  Having this
readily available seems to work better than cramming more information into
orientation sessions, or extending training programs, and can put everyone on
an even playing field.  Also, this is a great way to offer extended knowledge
as guides can thumb through them during slow periods and some have even been
inspired to do further research to contribute to the notebooks.

Incidentally, and just as an aside, I've found that security guides often
have a much more nuanced and complete grasp of objects than docents.  I
suppose it's because they spend such long periods of time with the objects,
and hear all the comments, (right and wrong) about them.  Often the guards
will doublecheck some of those comments with curatorial staff so they can
keep the record straight for visitors who may query them directly.  If I have
a question when visiting a site I usually try to ask a guard.  

Good luck on solving your staffing dilemma!

Christian Carr
Ph.D. Candidate
Bard Graduate Center for Studies in the Decorative Arts, Design and Culture,
NY