Congrats! We have two shelter rescues, an American Stafforshire Terrier and an English Bull Terrier. Both had histories of being neglected/abused. It's hard work to earn trust but can be done. You're already doing it right, speaking softly, praising, etc.
If you're still interested in adding another dog to the family, be sure to research both breeds for their tendancies with aggression or lack of, with other animals.
Also be sure to socialize them together a great deal before bringing another home. We're lucky in that our Staffie loves other animals, including all cats. The EBT is known to not really like other animals, especially those of the same sex. Ralphie (our staffie) is so calm and easy going, that Maxine (EBT) did great with him from meeting number one. It also helps to have both sexes and not a male/male, female/female combo.
Don't let him lick his incision!!!