Whenever I've introduced younger chicks (usually around 7+ weeks old) to my adults, I house them beside each other so they can view and visit through the fence, then I put them in the adult coop at night and close the door so I can watch them interact in the morning. Usually the little guys get pushed away from the food and water, so I just make sure I have multiple available to them and it usually isn't long before everybody knows their place.
I've never had blood drawn. A few feathers ruffled, but nothing serious.
I introduce after the chicks are adult size -- usually about 14 weeks. However if the birds are free ranging and have plenty of space to get away from each other, you can sometimes do it earlier. Be prepared to intervene if it doesn't go well.
I often find it helps to do the introductions on neutral territory, e.g. a new pen, or to introduce the bigger birds into the pen with the younger ones (so the younger ones have a territorial advantage).
You might also try sectioning off part of the pen so the birds can see but not touch for a few weeks, that often helps.
Thanks! I should've mentioned that the baby chicks have been in a coop next to the big girls for about 3 weeks! So they have been eyeing each other for awhile now!! I hope all goes well when I put them all together!!! Thanks again