Congratulations on the new addition.
I introduced 2 kittens to my then ~5yo tortoiseshell Tori 11 years ago. It took awhile for her to accept they weren't leaving. She then made certain they knew who was in charge (not me, her).
Fred, now 14+lbs and my biggest cat, still lays down and assumes the don't beat me posture when she walks by.
Tori - 16 now and diabetic for 6 years
Barney and Fred, 2 and 3 lbs at adoption, now 11 and 14+.
Fred loves bags and boxes, current favored box is from Pelican.