I understand very well about being worried about your current kitty being mad at you! But, it is a very normal concern and most of the time, it is a useless worry. You are so careful Robyn - thinking of everything!! I hope your mom is very proud of you
Just take things gradually and give Tigger LOTS of loving and time to adjust to the "intruder" (that is what I am currently calling the kittens ) Hissing is probably to be expected so don't worry.
I have previously brought only adult cats home and there has never been a problem. Some introductions took 10 minutes and others took a week or so. Each one is different. Don't worry, be relaxed and I have always felt the cats can sense your own relaxed attitude.