Tips for introducing adult cats into a new environment with a kitten?

I’m moving in with my bf and we have a kitten together. My oldest is 6 and I had him since he was 8 weeks. He’s always lived in the same home. He’s also the most territorial of all my cats, so I worry about him the most.

I purchased these calming cat treats. I was going to get a calming pheromone collar but they all contained lavender (toxic). I’m hoping that will help with the 3 hours of driving and hopefully meeting their new brother.