I have to agree with Pits not being pack oriented. I had Bo first and I was the center of his whole world. Now he is living with three people and three dogs. He is very friendly unless shown agression. The other dogs are Lil Jon (a 4lb Chiauau)and Lucky (a 45lb Pitt Lab mix) all are intact males. Bo doesn't even know Lil Jon is canine and Lucky is younger but can't keep up. No issues whatsoever other than Bo putting Lucky in his place when tthe stray showed up. No violence at all.
I guess adding a second dog is up to you. I'm looking for a female for Bo but i hear mixing sexes is a non-issue