Fighting or Playing?

Hi, I’ve been watching some videos on the subreddit to see whether my kittens are playing or fighting. But I’m still not sure so I wanted another opinion.

My resident female kitten (RK) is a three months old torbie that I’ve had for a month. My new male kitten (NK) is a six months old orange kitten. We got NK about a week ago and we’re going to do a slow introduction. But we live in a small apartment and it was impossible to let them smell/see each other.

We noticed them playing under the door with their paws so we recently started letting them play together under supervision. I think they’re just playing but my boyfriend is a bit overprotective, especially when we see RK with ears a bit flat, NK starts hissing/growling a bit, and fur is flying. So I wanted to see if you guys can tell if they’re playing or fighting here.