While particular breeds can be more cat friendly than others, every individual dog is also different so it;s hard to make any predictions and there are no fail-safe ways to approach this. As you will know from having your German Shepherd, it can take months to years to acclimate a dog to cats–and any dog that has ever shown any kind of real aggression should never be considered safe with cats anyway and always needs to be supervised. Many dogs will kill a cat if they have the opportunity–but the opposite concern is that dogs can end up losing an eye as a cat can easily blind a dog in an instant. We do have members here who have had their dogs blinded in one eye by a cat–so this has got to always be a concern.
It is much easier to train a puppy to live with cats–and much harder to try to train an older dog, especially one that has an ingrained behavior, or a high prey drive and a tendency to want to chase. It may actually be impossible–or, with patience, over time the dog's attitude may change. It is very hard to predict, but I think one of the things you need to do is to clarify whether you actually have an aggressive dog or just an excited barky dog as the 2 can be quite different. Lots of dogs get excited and bark at cats but that isn't really aggression. Real aggression is almost impossible to train away from and as noted, a dog that is truly aggressive to cats could never be trusted alone with them and would always need to be carefully managed in their presence.
I do have a post all about acclimating cats and Cavaliers–with lots of links on how to train dogs, in the Library section. That is probably a good place to start. A basic of doing this kind of thing is that it isn't just a matter of having them both in the same room, but of actually shifting the dog's perspective to one of very positive associations when a cat is nearby. So for example, the classic way of doing this is to offer small treats so that the dog begins to associate positive rewards for just being in the presence of the cat (this would begin with the cat at a very good distance away, not something as hard for the dog is having the cat just a couple of feet away). Over time, the dog or the cat could be brought closer, still giving the dog a steady reward of little treats. In general, this is the exact same approach used to calm reactive dogs on the lead, which will work but it takes time and can't be rushed. If the dog is overwhelmed–by another dog being too close, or a cat being too close–then that can reverse all the training up to that point because suddenly they just forget to be calm and go ballistic.
When I was running rescue, this is a point I would always make to people who would ask whether a dog was “cat friendly”. It is really very difficult to know this even in a household where there are cats around as dogs can be completely different with different cats. Often they are fine with their “own” cats but will bark their head off at strange cats at a distance. Mine are like this–however, I just brought a new cat into the household, and they were interested but indifferent about a cat that was inside–they seem to in their minds, differentiate between their “own” cats!
Best of luck–all I can say is this is something to do very carefully and cautiously so that no harm comes to either party. I will say that time does tend to help, and that one of the problems is often owners trying to force a friendship or even tolerance way too fast for the dog, which typically needs a much slower adjustment.
