You are correct in that kennel cough is due to one of a number of
respiratory viruses which pave the way for later infection by Bordetella
by "knocking out" the local immune reaction.
 
While Bordetella can conceivably (but very rarely) infect other mammalian
species, it is generally pretty well conserved to dogs, and poses little
threat to ferrets.  Ferrets are also not sensitive to the common viral
infections that facilitate kennel cough, with the exception of one of the
rarest, canine distemper.
 
Long story short - nothing to worry about with the current precautions you
are taking.
 
With kindest regards,
Bruce H. Williams, DVM, DACVP
 
Join the Ferret Health List at
http://groups.yahoo.com/group/Ferret-Health-list
[Posted in FML issue 3435]