There is no real way to answer that not personally knowing Pebbles. The way I do it is when the kittens are getting going on their own (so 4-6 weeks-ish) I will take them one at a time into the room where my resident cat is. I hold them in my hand with kitten head facing me so my cat can sniff the kittens butt. This is a non-threatening way to do cat-to-kitten introduction. My cat can smell without worrying about little kitten moving around and spooking her and if my cat does act aggressive in any manner then I can easily and quickly pull kitten away. I do this a few times with each one and then start setting them on the ground so my cat can watch them move and they can explore a little. I keep a very close watch during this process. Usually by the time they are really romping everyone is comfortable with everyone else, though I usually still keep my cat and mom cat separated.
Keep in mind my cat is used to kittens- I have had fosters since she was 10 months old so she's grown up with it. Only you know Pebbles and if she can be cat-aggressive. If in doubt don't introduce them. If you have to because you are keeping the one, then I would go very slowly just doing the in hand smell introduction for a few weeks. Until you are certain they are safe together do not leave Pebbles unsupervised with any kitten!
