Not being the current owner of a ferret but having kept rats (a kind of
landlord friendly substitute)for the past 4 years the following may be of
help.
 
Rats, at least in my experience, are totally passive creatures, they don't
bite and are very clean.  I would certainly recommend them over all other
rodents as a pet, probably males more than females as they are extra docile.
If you decide to get a rat and try to get it friendly with your ferret I
would go for a young male, don't get more as one may believe it was a small
ferret, two would soon figure out something was amiss.  I would definitely
cage the rat, even in a 'ferretproof' environment as rats love to make
bedding out of curtains or any other fabric that happens to be handy, they
also like to eat plants.  We keep ours on a shelf in an open fish tank, they
soon learn to jump out of the tank and onto the shelf above which has toys
for them to play with and sleep in etc.  they have never attempted to escape
from the shelf as they are apparently scared of falling (unlike our kamikaze
hamster).
 
we have introduced our rats to our friends dog and they have ridden on its
back, with some trepidation although I'm sure that given time they'd get
used to this new playmate.  We have also tried to introduce them to gerbils
and the hamster but these seem to be territorial and didn't approve of the
rats presence at all, the rats however were quite content to have a sniff
and get on with life and made no attempt to return the ill feelings.  So I'm
quite sure that if introduced gradually rats and ferrets would get on fine,
but obviously keep them under close observation for the initial stages at
least.
 
As a brief note, rats are susceptible to cancer, live about three years,
like human company, and love to ride around on your shoulders.
 
Good Luck.
 
Paul Webster - Exeter, Devon.
 
[Moderator's note: There are rats and there are RATS.  Ever been to NYC? BIG]
[Posted in FML issue 1607]