Your girls are soo cute! Congrats, kitty mommy!
We have introduced 3 kitties. We have 4 in all. They were all strays and pretty much grown at the time of integration.
For each introduction we have confined the new kitty to a room in the house for about a week at least. That way the new kitty can get used to the house first without the anxiety of meeting a new kitty too. This also helps the kids get used to each other, as they can sniff at each other under the door. We also start putting their food dishes by the door and feed them special treats, so they eat together and associate it with something good (but are still seperated by the door).
When we are ready to start introducing them (around the end of the week), one of us goes in the room with the new kitty and one stays in the hall with the original kitty. Then we open the door slightly so they can see each other. I'm not gonna lie, at first there will be some hissing, but eventually they get used to seeing each other. We do this off and on for a day or two.
When we finally let them mingle, we are prepared with a glass of water (last ditch effort to break up a kitty fight if things go bad) and some treats. We just keep talking to them, pet them and try keep them calm (keep the TV and stereo off to avoid adding to the confusion).
Even after they start to mingle, we still keep them seperated at night for a couple of nights.
That has worked every time. Good luck.
My name contribution if Nermal doesn't stick: Pesto
P.S. 2 female cats are the hardest to integrate so it may take a bit, but they will get there.