It looks nice. You obviously put some thought and work into it.
I have not raised Cornish Cross at all, let alone mixed with standards. There may be issues there. I just don't know.
I kept 28 chicks (breeds in my signature) in a brooder that was 5' x 3'. At 4 weeks, it was about time to move them out into something bigger.
Have you put a thermometer in there to see what temperatures you get in the brooder? The 90 to 95 degrees Fahrenheit for the first week is a range, not a set in stone law, so you do have a little leeway with that, but they do need it fairly close. I found with mine that some preferred it a bit on the cooler side. None wanted it warmer. I'd suggest moving the lamp to one end of the center section instead of dead in the middle. That way you will get more uneven cooling in the brooder so they can better seek out their comfort zone.
Thanks, I havent even turned on the lamp yet. I just set it there for the pic, but I'll try the placing it on one end idea. I dont know how the meat and egg birds will mix, but hopefully they can get along for a few weeks before I can seperate them into coops.