The biggest concern I have is with heat potential. I have a (hard) plastic greenhouse, with windows you can crack open. I usually leave the door wide open as well. Even with all that, on a sunny day , it can get to over 100 degrees, even when the outside temperature is only in the low 70s. Even for growing plants, I can only use it in the winter, as it will start to kill plants as the seasonal temperature warms.
Your greenhouse might be different and easier to keep cool. But, before I went any further, I would open all the windows as wide as they go. Put a thermometer inside at ground level and monitor temperature, particularly when you get a sunny day. Note the difference between the outside temperature and the greenhouse temperature, and try to figure out how hot it might get during the time frame you want to raise your meat birds. It may be cool enough now, but a spring hot spell could literally turn your birds into broilers.