Re: New to the site - Quick ?
I would think that he is about 5-5.5 months old, based on him loosing his puppy teeth.
Was he neutered at the shelter you adopted him from? I think he is a little too young to show "interest" in your other dog based on her gender. Mounting can often be about dominance and social order, rather than sexual, if that is the "interest" you have seen him show her! The worst "humper" I have ever seen was a female dog.
He sure looks purebred to me. One of my two GSD guys is wonderful with our cats, even tiny foster kittens. But one of my other dogs, a Pug, is **** on wheels with the cats. Teach your puppy from day one what is acceptable, respectful behavior with the cats and he should be fine with them. Don't encourage him to stand up for himself at this point, because it might get out of hand when he is bigger and older.
He might squat to pee for years. My almost 5 year old GSD still squats most of the time.
What plans do you have for training? I so miss having a young puppy in the house to work with! Your guy is at a great age! I envy you all the fun!