Even in summer they will need heat untill 5 or six weeks of age. By then it may be getting chilly at night so make that 6+ weeks. Assuming you don't have blackhead problems in your area they then would be ready if you are going to mix them.
Others may consider it a bad idea but I am currently raising a group of poults and chicks together that will probably stay that way. The chicks are being given the higher protein poult diet and heat. They are the biggest healthiest cochins I have ever seen and love the turks.
Poults look "big" at 6 to 8 weeks but are more fragile than chicks. They likely would get picked on by adult chickens that look about the same size. Be carefull.