I have 4 chicks that are 11 days old, I expect another 3 to hatch in 3 days, would it be safe to add them to the brooder with the older chicks? If not I need to set up another brooder. Also, my 11 day old ones seem to be quite comfortable without going under the heater plate (I have it raised from the beginning, so they have more room), can I just use a reptile heater for the older chicks and the heat plate for the newborns?