It is so easy to misinterpret reactivity as aggression. The reality is that reactivity is a tool that insecure dogs use to scare away something that is making them uncomfortable. As much as they bark, growl, lunge and carry on, there's often little to no follow through, meaning it rarely goes beyond a lot of noise or a slight nip.
I was so confused by my Toby's behavior when we first got him, it just didn't make sense. Some times he's seem shy, but friendly, other times he'd loose control completely and lunge at the end of his leash. But it all made sense when I really understood what 'Reactivity' means.
You've done wonders with Diego by being patient with him and teaching him that people can be trusted, now you have to do the same thing by introducing him slowly to your other dogs.
Since he seems so highly uncomfortable with other dogs, I'd start the training outside. Don't try to introduce the dogs, just have them in the yard together, spaced apart as much as your yard will allow. If your yard is too small, taking him to a park or somewhere quiet with the dogs approx. 50-60ft apart.
When Diego is being reactive or even if he's just fixating on your golden, redirect his behavior. Call his name, take a few steps in the opposite direction. As soon as you have his attention, even if its for a second, reward him. The point is to have him focusing on you and not the other dog. This will also teach him that good things happen when other dogs are around. Ask him to sit, lie down, look at you and any tricks that you've taught him, anything to help him get his attention on you and working and not worrying about the proximity of the other dog. Be sure to use high value treats.
When Diego stops focusing on the other dog, move the other dog a few feet closer.
You're going to have to be very patient with him, this whole process could take hours, days, weeks. It really depends on Diego.
I've found this blog to be extremely helpful, I'd suggest having a read through. This person's dog Maisy has similar reactive behavior around other dogs. She offers some great training tips for handling reactive dogs.
The first entry is at the bottom of the page. Be sure to read the one entitled "So, What is Reactivity Anyway." Its very insightful. It was a real light bulb moment for me when I was dealing with Toby. I knew he was fearful some times but I never really understood just how bad his anxiety was, till I started reading this blog and watching his body language.
http://reactivechampion.blogspot.co...d-max=2010-01-01T00:00:00-06:00&max-results=8