I'm sorry, but in my opinion, MM is way WRONG in this case. You should NEVER raise layers and meat birds together! The feeds they need are just far too different! The layers can not handle the protein in the meat bird feed, and if you feed normal chick starter to the meat birds, you will suffer in the end with scrawny thin birds not worth the effort to eat. You're best option is to seperate them.
I too, had only one brooder, and had both layers and meat birds to put in it. Simply divide it with cardboard, or other thin material, or even chicken wire! Simple to do and then all your birds will be at their best. We divided ours with a scrap piece of peg board my DH had layin around. Yes, it means you will need TWO feeders, and TWO waterers...but the meat birds will benefit from their own feed, as will your layers. In a few weeks, you will have healthy, plump meaty birds for your freezer, and then you can take out the divider and let your layers use the whole brooder.