I've raised Cornish Cross for several years. I start all my chicks on a mix of start and grow/turkey feed. At 3 weeks, I feed just start and grown 18% protein. At week 7 I finish them on Turkey feed at 20% protein. I have had VERY few loss at those are within the first week after receiving the chicks. The average process weight of the last 2 flocks I've raised this way was 5.75lbs at 8 weeks. I CO-OP around 300 birds a year, and since I switched to this feed schedule my production has been a huge success! The processor where I take them always rant and rave at how big they are, and how pretty they process.
As for covering them in cold weather, I do this when temps dip below 35, VENTILATION! is key make sure the tractor has plenty of fresh air flow, I usually leave a corner out of the direction of any wind open. Usually at week 5- 6 I move the tractors twice a day, this not only keeps them on clean fresh grass, but also reduces the possibility of introducing Cocci, E-Coli and Salmonella. For brooder, I would not keep it outside at all, I have a room in our barn that has my brooder boxes in it, I have a heater in there to keep the temp constant, I use only brooder plates, never a heat lamp. GOOD LUCK!