Emilio,
 
The problem you are referring to is not specific to the molluscs of course,
but probably exists in almost any taxa. There is a nontechnical discussion of
this by John Maynard Smith in a little paperback book he wrote called "The
Problems of Biology" (1986). Using the amphibian order Anura as an example,
this is how he summarizes the issue: "...whether the Anura constitute a
family, an order, or a class is an arbitrary matter: the point is that they
do constitute a natural group." Therefore the aim is to establish the natural
groups and the evolutionary relationships between them. But that is not
always an easy task. Personally, I don't pay much attention to the groupings
above the genus level until more phylogenetic information becomes available.
Even at the phylum level there are problems. Where do you put the
tardigrades, for example? Are they really a distinct phylum or part of the
Arthropoda? It is also possible that a revolution may sweep biology and
replace the Linnaean taxonomy (see the news article "Biologists urged to
retire Linnaeus" in Science 12 July 1996).
 
Have Fun!
 
A.