Hi! I think that I was Abby's foster mom! (I just searched for Carolina Boxer Rescue to try to see if any other volunteers, or adopters, etc were on Boxer World.) It sounds to me like you are doing the right thing, and I bet it will get better with time. Does Tabby come up to the crate while Abby is in it? If he is brave enough that might help Abby get used to him. For the crate problem you could try giving her something fun while she is in there (like a kong filled with treats and peanut butter).
I'm so glad that Abby got such a great home - hopefully she'll learn soon enough that you guys have pleny of love for both her AND Tabby and she doesn't have to take him out!!