I would hesitate to adopt him with having two other smaller dogs. I know some people here live in a "crate and rotate" situation, but I'm not sure I'd want to.
It's great you are researching, something I didn't do before I got my shelter "pitbull."
Thankfully he and our original dog are the best of friends. He is 2 now and our other dog is a 4 year old female hound mix. He does enjoy other dogs when we are out on a walk, big or small, although I always approach a new dog with hesatation. This isn't always the norm though for the breed. Of course mine is not a purebred APT. It took quite some time for him to adjust to our cat, who loves him now, and he loves her equally.
I still keep them separate though when I'm not around.
I would just hate to see the new puppy be sent away if he didn't get a long with your two smaller ones, or if a fight happened.
Of course if you feel you can handle it and are in it for the long haul, then it's up to you of course!.