//"I mean, once he gets use to the dog, he'll be fine with it"//
this is true of my dog-aggressive dog, as well, and of many other DA dogs whose owners are actively working to desensitize their dog. We CAN desensitize even DA dogs to an a particular unknown dog, one dog at a time.
It is the unknown dog that sets off a DA dog, and it's not *all* unknown dogs.
My da dog will love some unknown dogs on sight. But *most* he wants to argue with, if allowed, Buddy would lunge, growl, snarl, and eventually bite and fight if he could. (but he's not allowed, lol).
//"which I loved seeing, when we were at the kennel the other weekend, at least three dogs and their owners came in after us and Rucker didn't react. He noticed them, but didn't do anything."//
YAY! This is amazing!! I LOVE moments like this, too!! I have a handful of moments, where Buddy has stunned me.

ONce i walked Buddy down a Very narrow hallway(is a trigger for Buddy)
and crowded with german shepherds (Buddy loathes all GSDs),
and buddy calmly walked down this hallway, actually brushing sides with GSDs, one after the other!!!.
My heart was pounding, my hands were shaking, i was literally sweating bullets
so much for
"if the owner is anxious, the dog will react"
cuz
i was nervous wreck til we got outa
that hall!!! so this isn't true in all situations.
(my dog has also reacted when i was so calm, that i wasn't even paying att'n, etc, lolz)
I almost fainted from shock.
(obviously, this was NOT a planned event, we got trapped by a buncha ppl who have zero concerns that DA dogs do exist and just came down that narrow hallway that we were in)
I HOPE YOU GET MORE AND MORE OF THESE MOMENTS! It sounds like you are doing a great job with Rucker!!!
Has Rucker ever done anything besides bark and maybe leap up/pull on his leash
when he sees unknown ppl and unknown dogs?? If that is all Rucker does, it might be he is not truly a dog-aggressive dog, it might be simply excitement, especially if he has led an isolated life til now.