My brooder raised chicks typically start roosting around 10 to 12 weeks old. I've had some start at 5 weeks, some take a lot longer, but 10 to 12 is a decent average. This is when they are not in with adults. If my brooder-raised chicks are in with adults it is a different story. I suspect the way my grow-out coop is made affects that some. Until they start to roost mine do exactly as yours, sleep in a group on the floor, usually in a corner. From that perspective I think yours are acting perfectly normal, but I also think you know that from your post.
Now to your question, will them sleeping against that metal cause them any problems? Where do you live? How cold does it get? Metal is a great conductor. If it is pretty cold you might run into a problem. If it is below freezing I'd certainly be concerned. They probably sleep in the same corner every night. In colder temperatures I'd probably want to line that corner with wood to insulate them from the metal. I don't have a good definition for what colder really means though.