Back in the last week of June, 2003, I added a new kitten into Shadow’s realm. Shadow had been a solitary cat living with a single guy for 4 years. Last year, I added my (ex)wife and her daughter into the mix. It took Shadow a bit to get used to them, but he took it in stride. They have been in the house now for a year and he was working well with them.
The first 6 weeks, the kitten, now called Miss T, for Miss Trouble, stayed in a smaller room in my house, while Shadow kept his free reign. Now, though, they are more like a big brother, little sister pair. They still fight, but it is more playing than dominance. Miss T tried to be dominant, but Shadow, and his 12 pounds, kept her where he thought she should be, secondary. He still owns the house.
In the future, I think, if any other new additions come into my house, I am going the have to let Shadow approve them first.