Trying This Again, My Anti Social Dog.?

I have a 6 year old Parsons Russel Terrier who we got from a breeder at 15 months.. he was supposed to be a show/stud dog so he wasn’t neutered until about 18 months.. anyways, although he spent his first 15 months living with a breeder with other dogs he is VERY afraid (not the best word) whenever dominant dogs are around and sometimes fights with submissive dogs. To be honest we haven’t done much to fix it in the past since its never been a problem. He was able, at one time, to be around other dogs without a problem, and did fine in his training class of about 15. Otherwise he is a saint and listens to everything i ask of him. He is much better around female dogs then male, but basically they can all be problems.
Anyways, right now my family is thinking about getting a second dog (not a puppy, probably an older (4-6 year old) greyhound) and we’re worried about how Ace will re act to this, besides him being very attached to me we don’t know if he will ever get used to having another dog around all the time.
I am the his primary “trainer” and do lots of work with him, along with providing the largest part of his care so i would like to work on getting him used to other dogs before we try to bring one into the house. Any tips on doing this?
-i am willing to go to a pro. trainer to help this, but i’d like to try it on my own first if at all possible.
We went to a training class from a very reputible trainer when we first got Ace, i did all the work with him and still continue the training today.
I understand that this may be bigger then me, but i also believe i have the ability to do it. I am a horse trainer so i know how the training process works, and i’ve been perfectly capable of teaching Ace everything that he knows, including starting agility on our own. He is not a vicious dog and understands that i am the leader at all time, he listens to me whenever i command his attention. When around other dogs he looks to me before acting on anything.
I am NOT the classic Y!A person hoping for a quick fix, i know that this is not going to be an easy thing to fix, i know we should have delt with it before but it didn’t effect us then. I am willing to do anything to fix it at this point (within reason) but i can’t go back in time to do it.
If you look at this page: http://k9deb.com/nilif.htm
you can see the training method that i use, we follow every one of these practices everyday, at all times. He is not “out of control” just not able to socialize with other dogs.
-i am only repeating this question to better clarify our situation, and my hopes of at the very least starting this training on my own. I pride myself by doing all of my dogs training by myself and would, at the very least, like to see different methods of how to help his problems.