I adopted a Pyr puppy (female) three years ago and adopted a male a year ago when he was 18 months. They are spayed and neutered. They have gotten along famously, playing together and really enjoying each other's company. The male is more laid back and velcro, while the female tends to be much more aggressive in play. She is a few pounds bigger, and chases him to grab his tail and pull it. Consequently, he runs with his tail tucked under and looks really silly. They are house dogs and enjoy a fenced 1/2 acre lot. Since they use each other's collars as "handles" to pull each other around, I had to remove the collars when a recent scary fight occurred. We believe that the one who had hold of the collar got it twisted and was unable to let go, thus choking the other dog, creating panic and a very ugly fight with no lasting injuries. I grabbed Coleene's tail as Jim grabbed Bone's hindquarters and we more or less pulled them apart. A few seconds later, they were all happy again. When Bone turned 2, he started barking the moment he went out - not cute at 5:00 AM. He had matured to where he knew it was his job to "patrol". He does pretty much stop when I tell him to, and "treat" sends him flying back into the house. My point is, as they mature, the barking increases as part of the nature of the breed. So does their aggressiveness increase with getting older - the brain isn't a puppy any more. As much as Bone is easy going, an intact male gave him the evil eye in obedience class, from across the room, and it was clear that Bone wanted a BIG piece of him (a sheltie). Bone was riveted on the Sheltie, and I'm quite sure he would have attempted to take him out had we finished out the classes. Yet, Bone allowed a female puppy that he already had met to jump all around him, including pouncing on his head repeatedly. When I take him for walks, he will go right by a barking dog without reaction, and he allows my 14 year old Doberman (female) to rule him. She jumps around him, trying to get him to play, but he is actually afraid of her (for good reason!). So, I don't know if there are any absolutes when it comes to putting dogs together, and their behavior changes over time.
Darla & The Gang