Congrats on the new kitten. And you're doing the right thing with integrating the new-comer. It'll pay off in the long run.

Sometimes the cats with seniority just never quite get along with the new kid, but will settle into an uneasy detente. The growling and hissing will continue, but abate to maybe once a day.

We had the same situation with Goober (our oldest, who died last year) and Harry (they youngest, at that time). She just never got to where she "liked" him. She'd tolerate him. But that was about it.

Now it's his turn... Harry (now the oldest) treats Gleep (the youngest) the same way.

Oh well... cats.

Brian