I will be walking the Camino Frances from late September to early November, and am wondering if the hostels will be so busy that I will need to bring my own sleeping mat and bag. Is a sleeping bag required anyways?
I would say you should take "something" whether that is a sleeping bag, a sleeping bag liner (fleece), or a sleepsack, or a fleece blanket. When you begin, the weather will be warm but toward the end it could be cold in the mountainous alburgues.
Joe took a fleece sleeping bag liner.
I took a Marmot Pounder Plus bag.
We were both comfortable and we walked the same months you are walking.
You will not need a mat.
The alburgues will give you either a mat or a mattress.