Well, since Bambi's Children takes place after the movie, it can not, but you can still suppose it would have been so if a sequel was made. That's up to your imagination.
As for the things that got left out in the first movie...no. In the movie, it does not excist. But since many things are glanced over and there are many time gaps, you could just fit the characters in all the same, like Feathervenom did with her version of Gobo.

Yeah but does this all exist in the movie, despite not seeing it? I don't mean in the books

Bambi's Children? After the first movie, it centers around Geno and Gurri (the fawns at the end of the movie), Faline and a few friends.
Gobo was from "Bambi: A life in the woods" and is actually Faline's twin brother, the events around him took place in the movie's time line, he just wasn't included for time reasons. That was Karus' case as well. He is actually a friend of Ronno and Bambi's actual first Rival for Faline.

But were those advents supposed to of happened in the Disney Bambi universe?

Well, since the books were obviously there before the movie, so I dare to say they are canon. However, since the movie is just based on the book (in contrast to the "Spirit of the West" series about Spirit: SotC, where it was the other way around), the book verse is not quite the same as the movie verse. For example, in the movie, Gobo doesn't appear to excist, and neither does Karus. Geno and Gurri also didn't get enough screentime to make it as offical characters on this site.
But it you want to make a fan character out of Geno, Gurri, Gobo, Karus, Rolla, Lana, Boso or one of the other characters that aren't featured in the movie, that should be fine.