A protective dog is, as always, mainly a matter of the right training - any dog can be protective, although I have to admit a Doberman inspires somewhat greater respect than a Jack Russel, at least to people who know little about dogs (I've seen JRTs playfighting with other dogs, they never give up - and my neighbour has a Doberwoman who is essentially a self propelled teddy bear).
Getting along with your cat would even be easier - most dogs, given the chance, get along with most cats. It's just that they're rarely given the chance, both by humans and cats.
So it boils down to: how much time and effort can you spend on training and bonding with your dog? If you're working, can you take her/him to work? You can't just leave him alone (with your cat doesn't count - YOU are the leader of the pack, not the cat, and dogs are extremely hierarchical) at home or in your car for a day, definitely not for the first year or so (after that, with training, it may be possible - but not nice to the dog. See above, a dog wants/needs to be with his pack).