Kouji was a year old when we took in our boys Kenji and Koda. She hissed and growl and spit and swiped for a week, and then suddenly one morning she started the day by hissing and growling and a half hour later she was plying with them.
When we got our dog, Kouji seemed to accept him from Day One. Though she had a habit of getting all PO'd when Dashiell took his puppy antics a little too far. The boy took a little longer to adjust to Dash. Now they all accept (or at least tolerate) him.
Will be interesting to see how long it takes for all three of them to accept the new kitties. We recently catsit for my sister's two cats for three days over the weekend. Whle Dashiell accepted the two cats immediately, my little munchkins we're less than thrilled (as seen in pics on the Mett My Kitty section).
Kouji's pretty adaptable. She pretty much goes with the flow, she may be a little ticked at first but then she eventually gets over it. The boys, on the other hand,....well, Kenji's "mommy's little boy" so he's really the jealous type. And Koda is a big chicken who will flex his muscles (and he's got him, he's a huge cat!) and then take off running.
When we got the boys, we made sure to give Kouji lots of extra attention and affection. And then when we got Dashiell we gave all three cats extra attention, and some alone time with mommy and daddy to cuddle and play. From experience from my three babies and even from the two cats I grew up with, as well as stories from friends and family who had a cat and got addtional ones, the cats always adjusted.
Hopefully all goes well.