The humping of the kelpie was probably koda trying for one up man ship and the kelpie allowed him, hence it went to play.....Kelpie is most like a friendly happy dog. This makes it easy for other dogs.
On lead dogs are often more aggressive, because they have the fear of not being able to run away, hence fear aggression happens often on lead.
Doing a lot of "focus on me" and "look at that" type training with great rewards when they focus on you can often get you passed this when it has not advanced to far. Working dogs opposite one another and passing with dogs in obedience mode is also really good training...Start a long way apart and work closer and closer and eventually get dogs to sit/drop/stand stay side by side. often if dogs do obedience and are worked in obedience mode, they learn to cope. I like giving lots of rewards during this.
Working with lots of steady dogs of all breeds/sex/size is really good....And keeping the leash as loose as possible all the time is very important.........I have no issues with the occasional pop, but many people hang on to the lead to tight all the time. Let it go...the tight lead is often what started the problem.......you still have the lead, so if anything does happen you have control. Also learn to read your dog, anticipate, anticipate...So if you see the dog even look, get your dog to focus on you. Don't wait until it has happened. I use calming signals, if I see my previously people and dog aggressive dog get on a slight aler, I call her name, she looks and I lick my lips at her and yawn.....it work so well with her, if she lets down i say "yes" and if I have a food treat reward or just tell her how good she is.....
There is also nothing wrong with quickly changing direction and once the dog calms go back...find the comfortable distance and initially work there, getting closer as the dog improves
wagging of tail does not tell you as to wether a dog is friendly or not.........there are tail wags and tail wags. Some tail wags mean aggression. It is different in different breeds and situation. Ears and face/mouth will tell you more.
Reading your dog/dogs is really important, but also knowing and being able to read other dogs is helpful....if dogs start staring we need to intervene, many people do not. I will block a dogs sight if they are staring, it is often the beginning.
Anyway...........Dogs do not tend to grow out of things...habits get established and sometimes if they have succeeded may even get worse. Fear aggression may turn into aggression, if a dog becomes bold and confident.