M.M.
Rob was one of the pack and now he is gone. Even when the lowest of the wolf pack dies in the wild the others take notice. So of course River is sad, one of his pack has disappeared. All he sees is "my pack". When my husband deploys my big girl (shepherd) mopes for a couple weeks or so.
Does Nolan have his claws? If so I'd be more worried about Rver. Cats are usually faster than dogs and can jump higher.
We make our dogs lie down and watch the cats drink from the water bowl, we also feed them in the dogs' bowls at first. In our house cats are Alphas over dogs and our dogs know it. But I have two shepherds and one was a stray.
We have one room with a kitty door, so the cats can escape to their litter boxes and food without dogs, and so my dogs stay out of the litter.
Nolan's been in the house a couple days, let him have is freedom. Baby gates work. IN one house we gated off the stairs and kept cat things upstairs.
Watch them for a while, but they will grow to be friends.
Our stray shepherd and our stray kitty are best friends. Kitty will allow the dog to put her whole mouth over his head. The other two cats beat her nose so she doesn't bother with them.