I remember your previous posts I think...
This dog is not safe in your home - I am a behaviour consultant, I work with difficult dogs all the time.
Take it from me, management always fails in the end - people who live with their houses split with dogs who must not ever meet, fuck it up in the long run. Sometimes its a genuine mistake, mostly its someone (the man of the house or the dogs actual owner) gets complacent and thinks the dog will be just fine, I'll risk it...
Sometimes its not them that fucks up directly, its a visitor, a family member, a kid - but someone always does.
When it happens, someone gets hurt.
Even before that though, living in a split house like that is horribly stressful, you need to keep TWO doors shut between the dog and kid or dog and other dogs at all times. That might be a crate door and a room door or two room doors.
That is not a relaxing and easy way to live for humans.
Now, really concerning, from the dog side of the equation - you and he don't seem to have a CLUE about dog behaviour - older dogs do not simply 'get snappy' - im living with a 16 year old dog right now, he is not 'snappy'.
However older dogs DO suffer more pain, cognitive dysfunction, impaired mobility and this causes fear and a loss of inhibition. This may mean they're far more likely to use aggression as a defense when they are in pain or fear pain or just can't get away quickly enough.
We mitigate that by treating for pain - doesn't sound like he's bothered to do this.
Dogs experiencing pain are much more likely to feel threatened by unusual things - eg. people taking strong medications, people who are in pain or who are frail and move oddly - that can show up in them being aggressive toward such people - aggression is a dogs way of getting more space/distance from something frightening. Some dogs will use aggression in a maladaptive manner - so instead of freezing/backing off whilst growling and only biting when cornered ie defensively aggressive, they'll approach and be offensively aggressive.
This is also likely in breeds we've manipulated to bluff or approach and tackle things rather than back off (so pitbulls, german shepherds, malinois, rotties etc etc).
Even if you can mitigate the pain, a dog who is kept isolated from the rest of the house is likely to not have his needs met properly, and the move would trigger more anxiety and insecurity and stress, so he'd need more attention and company, not less. Add in two smaller dogs when he has a problem with small dogs, AND a new human AND a baby... that is a recipe for total disaster.