You often hear about how Pit Bull types just have to be with people and Marley is no exception. I know three months in a shelter can be a long time and though there are some dogs who can do well invthat environment there are others who just can’t handle it. I truly believe Marley is the latter type.
I imagine that being away from his people and among so many others who were being considered for adoption took its toll. I’d like to believe it is that frustration that lead to him “becoming hard to handle” and “dog aggressive.”
Since we saw that his happiness over being pulled lead to his immediate change in personality in the “hard to handle” department we were hopeful that the dog aggression would have been a similar byproduct of his frustration and not a true reflection of his personality. We have, however, proceeded with caution for two reasons. First and foremost while he is undergoing heart worm treatment we want to keep him quiet so even if he were to be friendly we can’t really allow him to play. Second, well if he isn’t a fan of other dogs that can be managed but if he really was aggressive we’d again be risking the elevated heart rate, etc.
Seeing Marley at the vet’s office near other dogs gave us hope that he was pretty friendly but we still took it very slowly. A couple of weeks ago, he met Queenie, one of our adoptable dogs and although she completely ignored him, he was tail-waggingly happy to see her. Just a couple of minutes and he was as smitten as can be and soon frustrated by the closed door barrier.
On New Year’s Day we tested him with little Ori who is the owned and fabulous dog of one of our new foster moms. (And also the adopter of one of Ray’s littermates.) We knew Ori was a very good neutral dog who would be very willing to correct Marley about any transgression. Also since he had met a larger dog successfully we wanted to see how it would go with a small dog.
We are pretty much over the moon about All. Things. Marley. But this is just pretty darned awesome no matter how you slice it, right?
Peace-a-bull Assembly 
The only reservations that I have with your Post (having dealt with heart worm in Ray) is your apparent desire to see how Marley reacts with other dogs. It is nice that he seems to be quite social, but every new dog will increase Marley’s heart rate either through excitement or trepidation. We avoided any contact during heart worm treatment in order to keep Ray’s heart rate down. Remember that congestive heart failure only happens once.
Love this! So many things can contribute to a great dog, not being ready for other dogs. But with patience and guidance, so many of them find their way back! You rock!