We have 3 GSD's that we keep as house dogs. We recently adopted another adult male from the GSD rescue. We have had 2 bad fights with our other male. What should we do?

We raised two German Shepherd from puppies Male and female, neutered and spayed. When they were between 1 and 2 we adopted a 1-year old neutered male from German Shepherd Rescue who had been abused. After about four days of his being in a kennel our Shepherds accepted him. The three of them are now between 2 and 2?. We adopted another male (neutered) from the German Shepherd Rescue a couple of weeks ago and are not having the same success. Our male we raised from a pup and the new GS have gone at each others throats and had to pull apart. This was after 2 days. They got together accidentally. We kept them separated and about a week later they were in the same room again by accident and again went at each other and had to be pulled apart. Is there any hope that these two will ever get along? We try getting them used to each other by taking them outside with muzzles and leashes on but our male turns his head and will not look at the new male. They look at each other and wag their tails when one is outside and the other in when they can see each other through the patio door. Is there any advice you can give me or a possible solution for this?

Shirley