What a great service you are doing for this child! I may have misunderstood your post since I don't know the story, but here's what I was thinking.....
Maybe you are already doing this, but you could contact your state and getting "foster training" if possible and see what resources they offer. It is amazing how getting a "bad" start can effect children in so many different ways. Not only can foster training give insight on why the children act the way they do, they will give you good advice on methods of disciplining and positive rewards for children with these types of issues(neglect, abandonment, abuse, etc.) It can really give understanding, plus they offer support groups and can get you in contact with others who are going through similar situations.
Good luck with your new little one. What a blessing you can both be to each other!