[matese]

Welcome to YT and congrats on your new furbutt.
Is he neutered.
Did the prev. owner say if he is aggressive toward dogs, All animals or ppl or all 3?
He may be territorial because cats go all over even in his safe place.
Is he territorial over his food, toys his humans?
If he is aggressive towards all dogs I have read it is hard to change this behavior.
It is obvious that the prev owner loved the cats more then she loved this little guy, this "could" be a reason why he is this way. The cats may have gotten more attention then this yorkie needed. I am sure this little guy got scolded a lot over the cats. He "may" have a different attitude living in a home where he FEELS LOVED and not being scold. He "may" lose his aggressiveness if he gets the love and attention he needs.
You have him only a few hours, he needs time to adjust to his new owners, home, smells, schedules. Bonding and trusting with new humans does not always happen over night, it may take time, just have lots of patience and give LOTS of TLC, speak to him in soft tones, no scolding just soft corrections. Let him build trust in you. He may have accidents in his new home, his world has been turned upside down, he is confused and needs time to adjust.Treat and praise like crazy when he potties where he is supposed to. For everything he does right treat and praise like crazy, I am sure he is not use being told he's a goooood boy. After a month or so in his new home and new humans you will see if he is aggressive and territorial. Please keep us updated.