I was able to add some that were hatched on 5-7 to a batch hatched on 4-25 with no issues. I would put a test chick in and watch closely so you can rescue if needed.
I have had no problems with mixing chicks. My white rocks were 3wks old when I brought hone my 5 day red sex-links, they had no problems. A week later when I added the banty's and buff orp's the bigger ones actually cuddled the babies to keep them warm!
I mixed my 2 week old Barred Rocks with my five-day old Cornish X last night while I was cleaning out the BR's brooder. It took all of a minute and a half for some serious eye-pecking to start. Keep an eye on them and see what happens.
Well the feed store lied..... I got there to pick up these 6 day old chicks and what is in the box but chicks that are bigger than mine that are 2 weeks old. They have to be at least 3 weeks old.
I left them in the cat carrier in the brooder so they could see eachother before I mixed them and then after about 20 mins or so I let them out and all has gone well so far.
I dont think they had alot of food at the store though because these chicks ran right for the food and water like they hadn't seen it in days.