Thank you for wanting to take in a senior. That being said, I think it would be a mistake. You know your resident dog isn't friendly to other dogs. You will be bringing another "possibly dominant" male dog into his home. It could be a recipe for disaster. At the least, I think the senior dog deserves to enjoy his retirement.
You could always try, and I'm sure there are those who will be able to offer advice on introducing them. Just try to make the decision that is in the best interest of both dogs.
Natty Boh 6-27-12
Annie 1998 - 8-2-12 RIP