Hello Quin,
 
There has been much discussion on Conch-L over the past months on
many different methods for cleaning shells, and you can find a wealth
of information by going through the Conch-L archives.  However, just
to give you a brief direct answer to the points you mentioned:
Chlorox bleach - there are some mixed feelings on this, particularly
regarding certain groups of shells, like abalones (Haliotis), cowries
(Cypraea) and shells with mother of pearl interiors.  Nevertheless,
chlorine bleach remains the single most widely used substance for
cleaning shells.  It does a good job of removing any kind of organic
matter, which includes algae, periostracum, and bits of tissue left
inside the shell.  It does not do a good job on inorganic
encrustations like barnacles, coral, worm tubes, etc.  Still, even
these may have some organic component, and the bleach may therefore
soften or loosen them somewhat, so they can be picked off more
easily.  I have cleaned many thousands of shells in Chlorox, and have
not noticed any deterioration of color, even on long exposure.  During
the discussions though, a couple of people did mention specific
instances where they believe some color loss did occur.
Varnish - please, no!  A varnished shell is a ruined shell.  You
mention preserving the "hardness" of the shells.  That is not
necessary.  The hardness will endure, unless the shells are stored in
very damp or humid conditions.  In that case, molds, acids, and other
problems also come into play.  The greatest threat to shell color is
sunlight.  If shells are stored in a cool (or at least not
excessively hot), dry place, away from direct sunlight, they can
remain fresh-looking for many years.
Toothpaste - this is a new one on me.  You specifically mentioned
fluoride toothpaste.  If you are talking about improving the hardness
of the shell, the same way fluoride improves the hardness of tooth
enamel, I seriously doubt that this would happen, and as I said, it
is not necessary anyway.
Hope this helps a bit.
 
Paul Monfils