Is believing in the pledge of allegiance to the ruler part of the creed of Ahl al-Sunnah wa'l-Jama'ah? Yes.

This issue is the belief that there must be a pledge of allegiance.

For a Muslim living in non-Muslim countries, who is his ruler?

No, the ruler here is not, if you are in a non-Muslim country, the non-Muslim is not your ruler, but you obey the laws of the country, do not cause corruption, do not steal, do not cheat.

You abide by the laws of the country, but this is not your ruler.

The non-Muslim ruler.