re: German Shepherds as Pets (with cats, small dogs, and kids)

If you're set on German Shepherds and don't mind paying top dollar, there's a group of monks-- yes, monks (I think they are or were Franciscans, but are considered Eastern Orthodox)-- that have a renowned German Shepherd breeding program in New York. Waiting lists can be long and puppies are over $4k I believe, but they are both well bred and well trained.

New Skete monks website

On the topic of training, the monks also have a training program for overwhelmed dog owners who need to train/re-train their dogs (any breed I assume). And they literally write books on the subject. One of the more well known ones is:

The Art of Raising a Puppy by The Monks of New Skete

Not sure if any of this will be of use, but best of luck.

Also... regarding how your future dog might act with cats or other small dogs, I think the proper term you'll want to research is "prey drive", which will vary animal to animal of course.