you said you had a slide n lock top. was it not locked? or is it just not a good one. i use slide and lock tops, and always wondered if they could get out. one of them is very secure and i have no fears of the snakes escaping, but the second cage we have worries me cause it droops in the center. i would keep the ferret locked and secure. i used to own one named ODEN, and they are curious buggers. he may try to attack the snake, or as ferrets are play roughly with it, not intending to hurt your little guy. i think i remember reading about ferrets being expert hunters for small animals, so keep them locked up, and dont let them leave your sight if they are out.
good luck.. to you.