Are ferrets good with big dogs like German shepherd dogs?
Will they attack dogs?
Ferrets and dogs can get along very well, however if you already have a dog then consider his personality first.
Is he/she good with small animals in general. Would he attack a ferret?
It also depends on the type of ferret you get, as most probably will just want to play with a dog(sometimes they do get a bit rough when they play) while others might freak out (very rare though as ferrets are more curious then anything) and could either react by running away or becoming aggressive as they try to defend themselves.
One thing that is important to keep in mind is...A German shepherd is much bigger then a ferret so there is only so much a ferret could do to it before it turned around and kicked its arse.
If you do get a ferret, make sure you educate yourself properly about them and always supervise the play time between ferret and dog. Make sure you introduce them nice and slowly, so that they can get used to each other first.
I hope this helps. Let me know if there is anything else you would like to know. :)