Notice that The Fourth of July celebrates "independence". This is also the same day that Scamp get his independence from being a "house dog".
Angel's ears could be a physical metaphor her character: The folded one hides her true feelings away and the straighter one represents the facade she keeps up.
See Fridge Logic below regarding Scamp's age, but if we assume Angel is the same age as him, she'd be quite capable of having puppies by that time. Age taboos being nonexistent for dogs, it makes sense that Buster's so fixated on her. Doubles as Fridge Horror.
Tramp being part of Buster's gang (and Buster's friend) seems to clash with the first movie where he was a loner. But then, you recall what Tramp also told Lady in the first movie (that he has lots of families, but none of them own him). Like those families and restaurants, Buster's gang are another one of those "families" Tramp spoke of. He may hang out with them, but only for certain intervals at a time. Otherwise, they would've known that Tramp associates with humans who feed him.
Despite it being just about a year since Tramp left the streets, he's become an eerily model house dog obsessive about following rules and content to lay around on pillows all day. It seems odd that so much of the fire would have gone out of him so quickly, but remember that he and Lady have already had a decent sized litter of puppies. Jim Dear and Darling probably had him fixed.
Reggie, the borderline-monstrous stray we see as part of Scamp's initiation into the Junkyard pack and again during the final confrontation at the pound, only barks and snarls whereas all the other dog characters speak comprehensively. Reggie has been described as incredibly dangerous more than once, so that makes one wonder...is Reggie rabid?
Tramp at one point surprises Buster and the other junkyard dogs when he proves that he still remembers some of his tricks from his street dog days (by effortlessly opening a locked gate the others had troubles with). As mentioned above, however, it's only been about a year, perhaps even less so since he left the streets; why would he have forgotten anything that quickly?
Buster had an extremely low opinion on Tramp after the latter left the streets to become a pet. As such, he probably was under the spiteful assumption that Tramp had lost all of his tricks as a result of his new life. The others may very well have just been following his beliefs.
A case of Writers Cannot Do Math, most likely, but Scamp and his sisters should be much bigger. If they were old enough to be crawling around at Christmastime (as they were at the end of the first film), we can assume they were no younger than three or four weeks old. By the Fourth of July, they'd be between seven and eight months old. They might not be quite full size by that time, but they'd be darn close. note That is, of course, assuming that Tramp doesn't have ancestry from much smaller breeds and managed to pass that particular trait on to the entire litter. Seeing as all four puppies are clear copies of one of their parents rather than mixing their traits, it's unlikely anyone put that much thought into their designs. Angel seems to be the same age, but at least she has the excuse of most likely being a small breed (the Disney Wiki states her to be a Pomeranian mix).
Regarding Scamp, it's possible that he did, if fact, inherent a smaller size from his mother, who herself is of a rather small breed, and that Scamp actually is close to his full size. This would explain why he sounds like a teenager while still looking like a young puppy he's just smaller than his father.