They're all very mixed, but difficult to say what's in them.
I don't see any ethical or legal problem with taking abandoned, unwanted livestock. Definitely quarantine, as others have said.
Your big problem is going to be catching one of them. I don't foresee that being easy, especially if those roosters have been roaming free for a long time. Best bet is to go around the time when they would roost, and catch one roosting if it isn't too high up. Wear heavy clothes and gloves in case it pecks, and have a box or cage on hand. If you use a cage, have a towel or small blanket on hand to cover the cage with. Also, have its temporary quarters ready for it before bringing it home. You will need to check for lice and mites so he doesn't spread them to your flock. Treat if necessary.