Could you post a photo to confirm it is a Cornish Cross meat bird versus some other type. What I'd mostly be looking at would be body conformation and the legs. Color, how wide apart, and how stocky they are. A close-up of the head might be helpful to determine sex.
Your story is not that unusual. When I was in Northwest Arkansas, the headquarters of Tyson, trucks hauling chickens to slaughter were pretty common. Occasionally one would escape. A cage probably wasn't closed properly.
Sometimes that works, sometimes it doesn't. How much room they have can be important. Can it hide away from the others if things go bad? If you try that, be out there at daybreak as they wake up in case you need to protect it.
Unless you are introducing a mature rooster to a flock of all hens the see but don't touch is a safer way to go. That doesn't always work either.
That's mainly why I recommend against trying to keep that Cornish Cross with your flock. It is special, a rescue. You are developing a strong attachment to it. It is going to get huge and probably suffer a physical breakdown or, if lucky, a heart attack and die quickly. It's likely to suffer. Even if it just drops dead without suffering you will feel it.
It's just a chick, so it can be hard to integrate. It is a single chicken which can be hard to integrate. You will sympathize with it even more.
Some people just have to try. If you do I wish you luck.