bringing a new dog to the house

Question
Ok here's my question, I have a 5 yr old German shep. I've had him since he was 6 weeks old.  He grew up with my rott who we lost a yr ago to cancer. He has never been around other dogs except our rott. I did try to intro him about a yr ago to a saint Bernard who was already his size at 6 months old and it was kind of scary cause i was really worried that he would attack her cause of the way he was acting. Well we know someone who has a stray female and my husband would love to take her, I'm worried he will hurt her. Is there a proper way to introduce them without either one getting aggressive? My dog is very much protective of our family but not the greatest with strangers, he does show aggression which i believe is his way of letting people know that this is his territory. Pls help me, should we leave well enough alone or should we try to get him a playmate??

Answer
I am not sure how much good it does, but ''experts'' say to meet on neutral ground.  Can't hurt.  Was the incident with the St. Bernard at your home, or its?  Keep both dogs on leash or have one tethered.  Let them sniff each other over.  Try to relax as much as you can.  If you tense up, he may decide to do something about the dog causing it.  Give them more freedom, but not enough leash that they can circle each other and make a tangled mess of the leashes.  By selecting an enclosed place, you can try letting them loose if it seems to be going well.  

If it doesn't, perhaps you will need to forget her.  Usually a male, female pair has the best chance to get along.