Liners typically warm you about 10 degrees F, and silk and cotton do not make much of a difference. While the weather is getting cooler in September, the inside of albergues will still be "room temperature". I have always found the sleeping bag liner gives adequate warmth, and there are almost always blankets around, and they are as clean as the mattresses, pillows, and bunks! I sleep comfortably at about 68 F, so the liner is perfect in a 58 F room. I also have a Tyvek blanket that warms another 10 F, so the two of them give me comfort in a 48 F room. Typical lightweight sleeping bags claim comfort in a 40 F temperature, so I pretty much have that covered.
Evenings in September may require a fleece, although you can always add layers of clothes including your poncho. Unless you are fashion conscious, don't worry about wearing a poncho around town or in the albergue. Your objective is more likely to be a light pack than having precisely the right clothing choice. You can sleep in your clothes, and no one will be the wiser, but you will be the warmer for it!