Here is a great article to help with all assimilation:
Canine Adoption Advice
January 2006
By Mardi Richmond
Here's how to make your new dog's adoption work for life.
Adopting a new dog is exciting, wonderful, and a happy time. But bringing a new dog home is also an uncertain time. What will your dog be like? Will he be a good match for your family? Will he be everything you hoped for? Bringing a new dog into the home can also, quite frankly, be a rather shocking time for you and your family. Suddenly your life will be compounded by the energy and needs of the new family member. Everyone will go through an adjustment dogs and people alike. What can you do to ensure that you and your new dog will settle into a long happy life together?
The entire article may be viewed at
http://www.whole-dog-journal.com/issues/9_1/features/Dog-Adoption-Advice_15779-1.htmlAs far as actually introducing the two: separate genders are the best prospect, and helps to reduce competition so this is a good thing. If there is any concern about the Mastiff, always start side by side without nose to nose contact and walk parallel. As a result, you want to just bring your girl on leash up next to him and walk side by side. The two walking humans can be inbetween the two dogs as you walk if there is great concern, or just one human in between the two dogs to start. Eventually the two dogs should be side by side while walking if they can progress to this - keeping their attention and focus forward. If this sufficiently calms them, then you can stop and determine if they will greet politely (sniffing that is not too invasive or pushy, and no posturing (head held high, chest puffed up, body standover etc.).
If there is reactivity, aggression, or other issues involved, I recommend joining Pat Miller's training discussion list to get more guidance:
peaceablepaws-subscribe@yahoogroups.com