I don't have any personal experience with JRT's but as a dog afcianado(sp)
I think your right to be concerned (to put it mildly).  My impression has
been that JRT's are very close to their hunting heritage and have very
present high prey drives.
 
The fact that you have trained your JRT's to a reliable down stay is
wonderful and promising!  When the ferrets are caged do you let the JRT's
sniff around the cage?  How do they react?  You might try this and reward
them when they act calm.  You could also try the reverse.  Get an X-pen or
crate and let the ferrets run around - let the dogs see you play and
interact with the ferrets.  I would recommend a fairly long course of this
sort of semi seperated interaction.  You could also try giving the ferrets
a treat in front of the dog and then give a treat to the dog - ALWAYS treat
the ferret first to reinforce that the ferrets are "important" in the
hierarchy.  If your dogs are food guarders or food competitive you may have
to modify this.
 
Personally if it were me and I were transition to the not in a down stay
I'd use a muzzle and definately a leash.  Sounds extreme but from personal
experience let me tell you if the JRT's snap it'll be so fast you won't be
able to do anything about it.  I was sitting on a step, my dog right in
front of me, leg touching my knee, his head about 6 inches from mine.  My
ferret got up on the step, about 6 inches away from me, she lunged at the
dog, the dog snapped back, neither connected thank God because it happened
so fast even though we were right in a little huddle it was over before
what my eye saw reached my brain.  Had he really wanted her it would have
been a done deal.  Very scary.  I used a muzzle during the transition
period.  It was the only way I felt sure.  One other thing - I never leave
the dogs toys or food out on the floor.  I only play with the dog and his
toys when the ferrets are off and asleep - he gets too excited and I feel
like I lose a little control.  If the ferrets get up - the toys go up
(where they stay) and I immediately go into "settle" for the dog.
 
Good luck and be careful,
Mary, Booker the Aust Shep and Boris and Geisela
P.S. Please keep me posted, privately is fine - I'm very very interested
in dog/ferret interaction.
[Posted in FML issue 2484]