Himalayans tend to be exceptionally loving, loyal and intelligent cats. According to J. Anne Helgren, author of Himalayan Cats, it helps to get two Himalayan kittens at the same time. They can then keep each other company and develop a closeness that will likely last through adulthood. It’s possible to add a second kitten at any time, but an adjustment period will be required, since the first kitten probably has established its territory in your home already.
If you wish to add an adult cat to your household, Helgren advises not to leave the two cats alone together. That’s certainly important in your case, since the kitten will have a size and strength disadvantage. Ideally, the two cats should be able to see and smell each other, but not touch at first. Cats communicate with these other senses, so they will get to know each other a bit via odors and visual cues. Give the two cats time to get used to each other. The fact that you have a kitten should help, since the second cat may not feel as intimidated.