All of those things are really dog dependent. Some dogs seem to bond more than others, while some just seem to tolerate each other.
I added a rescue Chihuahua to my Dobe gang a couple months ago. She was your typical unsocialized chi - very fearful, used to being carried around. She was signed over to our clinic because the owners did not have any $ for the emergency c-section she needed. They agreed to sign her over to us, let us treat her and find her a home. I agreed to foster her and her babies and see how she fit in w/ my gang.
She has come a long way in only 2 months. The first night, she growled & snapped at me. The second day I dumped her out of her carrier (because she was guarding it), and gave her some food. She then decided that I was ok. She has gone from growling/snapping at the Dobes to now in the last week actually playing w/ my younger Dobe girl. She has started playing with toys, and is learning how to be a real dog that walks on the ground and has rules

She has decided that she loves me now. Now that she's vaccinated and no longer caring for pups, we can start to get out and get her more socialized. She is still VERY fearful of strangers.
My advice would be to just be patient and not try to rush things. Also to be sure to give your original dogs some extra love. Be consistent w/ your rules and enforce them from day one.
Good luck w/ everything!