Question About Introducing Adults

I am brand new to raising chicks. I have 8 that are somewhere between 6 to 8 weeks old. They are outside during the day and in the coop at night. We are deep south and our temps are well into the 90's and the lady we got them from said they were find to be out from under the lights with our hot weather. They are doing great. The thing is when I got them I was actually looking for some that were a bit older but she had only the babies at the time. Well this morning I have a chance to get three speckled Sussex that are adults - they just turned a year old. They have been raised with children and are docile. Would it be a problem to introduce these three adults that are already laying in with my babies. I need to tell her within the next few hours because someone else is wanting them. I can get all three for $20. My pen is plenty big I looked up the amount of space I need for that many chickens when grown and I have enough square footage and space to spare. We will need to do an expansion on the coop, but thinking about converting an unused dog house in the interim. I just need to know if it is a bad idea to introduce adults into the mix or is it okay,

At six weeks they are to young to be put with adult hens. If you have the space you could partition the coop to give the chicks space in the coop and give them time to grow up for another six weeks.