with lots of hay to snuggle into as well, then yes, it should be. The heatpads work well in providing a warm spot for them to sit when put underneath a hidey (particularly a fleece hidey).
i keep my piggies in my shed year round (although I always have the ability to bring them in if I can’t keep them warm enough). They are kept warm enough with thermal covers, snugglesafes and lots of hay, so if your piggies are inside, then then it’ll be easier for you to keep them warm.
but if your piggies are indoors, and if it was to get down to 10 degrees, then it’s going to be uncomfortable for you too. The rough guide is if you are comfortable, then the piggies are too. If you feel cold, then they are already struggling