Quote: ^^^ What Blooie said! ^^^
I raise Cornish X meaties for my freezer every year. I use a Brinsea Eco Glow 50 because I had it before this thread was started and because I usually have 20 - 35 meaties at a time.
They grow fast and have a higher metabolic rate (they are bred to grow fast so must be able to convert food to tissue quickly) so they get hot quicker than your other breeds. Like Blooie said, you need to accommodate that difference. The MHP is, more than likely, too hot for them. Turn the temp down and raise the roof. The hot bodies of your meaties will help keep the others warm. Listen for distress peeping to know if other adjustment is needed.