First, congrats!! ok, so they should be introduced outside of the home. take them for a nice walk that differs from Bella's normal walk route. then when you bring them into the home, let the rescue go in first so Bella does not think she is in charge or the leader. Is bella dog friendly? If so, you can let them interact under your strict supervision. IF you see bella starting to display any behavior you do not like, trying to show dominance, staring, etc.... you need to give a very firm, very loud, NO Bella command and she should understand her behavior will not be tolerated. When I brought Frankie into the house Brutus was not having it so I had to gate them apart for approx. 1 1/2 months, so Brutus could get used to Frankie being in the house and 3yrs later they are the best of friends. any questions please feel free to PM me or ask here in this thread. I will check back.
I jsut read that Bella likes other dogs, so maybe just let them get to know each other separtely until you get a better feel for the dog coming in.