The new cat is almost twice her size, and he is 7. He doesn't know a stranger, so even after the introduction period where I would let them sniff each other through the door, etc., he still runs up to them like they are his best friend and it unnerves them.

Like I said, she has stopped hissing at him, and they mostly tolerate him, but she has stopped coming downstairs for the most part and has definitely stopped being my huge goofball who would smack at the 3 year old's back paws to try to trip her.