I have three male dobermans in my house - a 10 year old, a 3 year old, and a 1.5 year old.
The 10 year old is a (likely permanent) foster. The three year old belongs to one of my tenants, and the 1.5 year old is my performance dog. My youngest was obtained first, the foster second, and the three year old last.
My house is a house full of no less than 4 dog crates (at least one on every level of the house), expends, baby gates, and leashes. It's a pain in the ass to live a segregated household, but it is what is required.
The three year old is not "my" dog. When he was adopted, it was with the understanding that he was going to come in second to my own dogs - I own this house, and my dogs take priority. If one dog is out while I am home, it is my own. Adding to this, the dog is heartworm positive so he is currently on strict crate rest anyway - but he will never be a "house dog" while I am at home (he is welcome to be out when I work, train, travel, etc). He is very submissive and fearful, and doesn't appreciate my insane male's advances, so they do not live "together". They can easily be in the same room, walk past each other, but they are not allowed to interact with each other without someone attached to them. They can travel, they can train, they can sit, stand and walk next to each other. But they WILL NOT ever truly live together. They will not be around each other off leash.
My older foster male is laid back and could care less. He is allowed to remain in the house uncrated when I am not home and is allowed to wander as he pleases. He gets along well with my young male and they are allowed to interact WITH MY SUPERVISION. They do not eat together. They do not ride in the car together unless one is always crated. They do not get left in rooms alone together. They are not given high value treats together. They do not sleep in the same places (my male sleeps in bed, the foster sleeps on the floor).
They do play together - in the yard or in my living room. My male is good with dogs (At this point - he is 1.5 years old) and he is responsive to their wishes and takes it easy on the old man, as Tyler is not 100% healthy. Could something escalate? Of course - every day that is my concern, which is why they are NEVER allowed to interact without me attending. I don't even go to the bathroom and leave them if no one else is around.
Above all, I know my own dogs. My youngest male is currently MARVELOUS around other dogs, but I know things can change. He does not interact without me observing. But me knowing them isn't going to stop a fight - it's seeing the warning signs if there are any, and MORE THAN ANYTHING ELSE - not giving them the opportunity to engage.
The tenant's dog is fearful and submissive, and he responds aggressively when cornered. He will never meet my young male loose, and they are never going to be more than housemates. For these two, we crate and rotate. even though they "see" and "meet" each other 2-3 times a day and have no problem, I know that the albino will respond aggressively and while my male will not start the fight, he will not back down when challenged.
My older male foster and my young male coexist peacefully for the time being. The older male requires much less of my time and is happy to have a few jaunts around the yard, a short walk, and cuddles with me. His lower demands and his peaceful nature make coexistence possible for now.
It's a pain in the ass for newcomers to the house to have to remember to replace baby gates, it's a pain not to be able to just toss all the dogs in the house out in the yard. It's a pain that the dogs CANNOT be left loose with each other when we leave, that they all need to be separated when eating.
But that is my life. And when I add my bitch into the mix, I don't imagine things getting easy - but every dog has what they need, every dog is trained, cared for, and loved. It just means it doesn't all happen in the same room at the same time
(it also helps that my dogs come to work with me every day, so it gives the albino male 10-14 hours a day that he has free run of the house if his owner is home). As my male ages and the older male does as well, I am sure that they will spend less and less time together. Right now I will enjoy that my male is passed out on the couch and the old male is sleeping on the dog bed at his feet.
My advice to you? Don't trust that just because he is a puppy and has been fine so far, that nothing will ever happen.
My BIGGER concern for you is that you have two pit mixes, don't you? That is a whole separate issue in and of itself - how old are they? I am involved with pits as well, and have some very good friends heavily nvolved in rescue, training and showing pits - and they all have one phrase that is repeated all the time -
NEVER TRUST A PIT NOT TO FIGHT.