First I would like to say you did a very humane thing rescuing these chickens. Now as to the possibility of disease, here are my thoughts.
Whoever it was that dumped these chicken most likely was fed up with chicken keeping. If they were diseased, I think the heads would have been removed.
To quarantine these chickens now would be pretty much too late. You already handled them and so if there was some kind of bug on them, your flock already has it if you went to them.. Obviously I am talking about microscopic bacteria. They may have mites or lice, but that is treatable. and not life threatening.
And finally my personal view,, The Creator watches over those that do good.
You would be surprised how chickens recover from trauma they experienced.
Keep us posted on their recovery.
WISHING YOU BEST