Your friend (and yourself) may like to read this (about the 'Alpha' role):
http://www.4pawsu.com/DebunkingDomMyth.pdf
Also this is interesting if you believe in the 'pack' methods of training:
http://www.4pawsu.com/267alphastatus_english.p df
To answer your question about fighting, yes it is possible with any two bitches. I know of a breeder of Great Danes, she took a bitch to a show, when she returned she let the bitch into the living room with her two other bitches - they started to fight, the lady had to have a vein taken out of her leg and put into her arm to replace the one torn out by the dog bite she received trying to split the dogs.
Socialisation does not come into it. If a bitch takes a dislike to another bitch you have major problems. Imagine if you (or your wife) were forced to live with someone you took an instant dislike to! Dogs are like people sometimes they just don't like the other dog for whatever reason.
I think you really need to think long and hard about getting this breed. Does your wife want the dog as much as you? After all it sounds like she will be doing alot of the work with the dog.
I have a giant guarding breed, he is now 12 and has totally taken over my life. He has pushed me to the absolute limit many times. I have been heavily involved in dog training for a lot of years and I thought I knew what I was taking on, I studied the breed for 10 years before I got him. I knew nothing!!
IF you get it wrong with a dog of this size there is really only one option. Put it to sleep. Retraining is not a feasable option.