A couple months ago I picked up some barred rock and buff orpington pullets to breed with our roosters. They are around the same age, but the buff orpingtons seem to be growing quicker. They are all very active and love to chase each other around the stock tank. They stay out in the garage where I leave a space heater on at night, but on days above 40F I leave the door open so they can get some fresh air.
Both of these breeds are dual purpose and excellent for egg or meat production. They are also both classified as "heavy" breeds which reach a size of 7-8 pounds.
In the next few months, I will be able to start integrating them into the adult flock. Special care has to be taken when integrating younger birds because the adults (especially the roosters) can attack and kill the younger birds. Isolating the younger birds near the adults in a smaller pen will help them get used to seeing and interacting with each other.