Hola and welcome to the forum.
Sleeping bag questions are hard to advice on, as all have different needs, depending on how sensitive one is to temperature.
Personally I need to not be cold at night to sleep well, so I brought a -5/-8 on a winter camino and was happy for this decision.
As it has been said, many albergues have blankets, as well as some albergues has no heat, so it varies from place to place, how warm or cold a night will be.
Here are some advices regarding sleeping bags:
• Use an inner silk or fleece liner/bag.
A silk liner adds about 5° degree celsius to the bag and a fleece 8°.
On winter camping/trekking two liners are often used as the effect adds up and the liners are light weight.
• The sleeping bag does not make you warm.
It function as a thermo and isolate you.
This mean that it contains the warmth/heat you put in, so make some gymnastic before you enter and you will sleep warmer.
Think of the difference of poring cold or hot water into a thermo.
• Wear a beanie, scarf, long johns, gloves, cloth while sleeping.
• Unpack your sleeping bag early before going to sleep as the down/synthetic needs to unfold to provide maximum insulation.
• Do use a sleeping bag that matches your size. A to large result in to much air that needs to be heated and a to small result in to little air to provide best insulation.
• Put some hot water in your 0.5 water bottle and bring it into the sleeping bag.
• Never go hungry to bed. Energy equals heat.
• Remember to go to bathroom and pee off before going to bed.
The body uses a huge amount of energy if you are in a state that needs to go to the bathroom.
Buen Camino
Lettinggo