How old do my chicks need to get before I put them w/ my older pullets

I just did what you did also and have heard that sneaking them in at night is how to introduce them too. The only thing that can be an issue is that the babies and the pullets can't eat the same food for awhile. This is the dilemma I am having right now...

I am following this post since I have the same problem except my existing flock is 5 years old and includes a rooster.  I don't know how to introduce them either.  I currently have them side-by-side in a divided coop where they can see each other but not mingle.  The babies are 8 weeks old now and have been outside for 3-4 days.  Also, one of the older pullets is kind of the "boss" of all the others (first to the food and treats) and she may not take to the newcomers very well.  Hope somebody with more experience responds.

I usually wait until the babies are roughly the same size as the adults. That way they have a chance of holding their own until the new pecking order is established.

Put similar sized fowl together so that they can stand their ground

I don't put the younger ones with the older ones until they are at LEAST 16 weeks... right now I have a number of 17 week old pullets that I'm not going to introduce to the big coop for another month.

In a couple of more weeks I am going to divide my run in half with poultry wire.  The existing flock will use their coop and 1/2 of the run.  The new chicks will use the new second coop and the other 1/2 of the run.  I plan to keep them divided with pultry wire until they are at least 18 weeks old.  The older flock has 15 one year old hens.  I have 25 chicks in the brooder (I think there are 3 roosters that will need to be rehomed in a few weeks).  This is way too many to try to integrate by putting the new ones on the roost of with the older ones.  My plan is to remove the poultry wire divider early on a Saturday morning.  I will then open both coops at the same time and cross my fingers hoping that integration will go well.  I will be watching all of them closely the first couple of days.  I am hoping that since they have seen each other everyday in the run for a few weeks the integration will go smooth.

I've had easy integrations and rough ones.  It helps if they are the exact same size, but, if for some reason you can't wait, try to give the little ones a place to escape that the big ones can't reach.  You could also, put two or three of the older hens in with the young ones for a few days before the rest get integrated.  Give the young ones had a place to hide, too.  I did that and that was my easiest integration.   
I'm not experienced with introducing roosters, though.
Good luck,
Dale-Ann