Well our rock was a rescue got him at 7 months, I was a Bully breed guy before hand and Rocky was a whole different world.
If your current dog is a female don't get another female.
The cat thing is irrelevant if you do it right, don't let the dog chase the cat...ever! Not one paw forward! If the dog is calm around the cat then the cat will accept the dog or ignore him, it's a cat! If the cat wants to engage the puppy I'd allow a little slack but still no "chasing!"
With only one dog you might not have an issue with this one but GSD have high rank drive. I never dealt with that with my other guys but that was my biggest issue with Rocky and Gunther two Dominate males. Gunther Bull Mastiff/Pit Mix apparently Roc had a "problem" with not going out the door first! My mistake and it showed up in the 18 to 28 month range.
People, company in the home, yep that was work, My others guy took to company like ducks to water ( especially the Boxer). Rocky not so much, more training and a soft muzzle were involved, He never bit anyone (never gave him the chance) but he clearly indicated that hugs and kisses were not on the menu for guest!
Overall he was a lot more work than I was expecting but he is a happy well adjusted dog that is safe with company (Roc go to bed and stay) and in public and around other dogs in public places. (No dog parks for us, so I've no idea how he would do in that environment but he's fine at the Vet office.).
He's still not a big fan of having company over but he's safe and civil in the home and pretty much aloof to strangers, good enough for me.
7 yrs old now.
I did all the hard work and he turned out just fine, so I would consider another GSD, my wife on the other hand loves our Rocky but she says "NO."
So it'll be back to Boxer world for us, for Rocky's next buddy!