My daughter's 4H birds were hatched on May 29, she received 47 birds. She only lost 3 birds, 2 the first week and the third at 5 weeks of age. We have weather ranging from the 5o's to the 90's.
If you provide adequate ventilation, water, and shelter you should not have issues with your broilers. After all, the majority of commercial broiler production is in the Arkansas, Mississippi, Alabama, and Georgia region which has high temperatures and humidity. If they can raise thousands of broilers in one building then I don't see why you can't raise 25 or 50 in your backyard without issues.
Jim