A. That all depends. There is a big difference between managing it enough to have that dog become part of a multi-dog home and eliminating it enough to have that dog become part of a multi-dog home. How much you can eliminate it & how much you'll forever have to manage it (with physical barriers & stellar obedience) depend on the individual dog... NOT just the breed. I am living proof that training (along with lifetime socialization & physical exercise) can make it possible to run a 4-male-pit-bull-household in which one of the dogs was on death row for liability reasons relating to his dog- and stranger-"aggression" issues.
There is also a real difference between true AGGRESSION and over-reactivity. Also a big difference between issues that stem from lack of early socialization/lack of exercise & issues that stem from genetics. Real, true aggression is really rather rare, and with truly aggressive dogs, my feeling is that you can usually get to a place where they are completely manageable, but you cannot completely eliminate every aggressive tendency.
Ya know, I also have to add that I've seen what people said was impossible come to fruition many times under certain circumstances, so if you find the right formula, almost any dog can be "made" to live in a multi-dog household. How people feel about "making" that dog do it is a different story.