What is the significance of the “six kings” in chapter 26?

Two of the major themes of Candide are the arbitrary nature of fortune and the universal nature of misfortune. Chapter 26 represents perhaps the strongest statement of this theme, as it carries these observations across to the most privileged subset of the eighteenth-century socio-political order: kings.
In this chapter, Candide and Martin are dining with six strangers, all deposed monarchs, who have come to Venice for the Carnival. We see here the former Grand Sultan, who previously deposed his own brother and has now been deposed by his nephew. Next, there is Ivan of Russia, who was dethroned as an infant and raised in a Russian prison. Then, we meet a former king of England, two former kings of Poland (though the second of those two has succeeded in winning a different kingdom for himself), and finally, the former king of Corsica, who now lives in a state of utter poverty, dependent on charity. As the scene ends, we learn that four more deposed monarchs have arrived at the same location, also looking to attend the Venetian carnival.
Here we have Voltaire taking his attack on social and political privilege to its logical conclusion. Even royalty is in no way immune to those twists and turns of fortune. In this respect, even the mightiest of kings is no different than anyone else. All are equally human—they must all face the uncertainty of the future, and their circumstances can change dramatically. Indeed, it's worth noting that at this point in the story, Candide (who, as we saw in the book's initial chapter, was driven out of Thunder-ten-tronckh for lacking sufficient noble status to enter a romantic relationship with the noble Cunégonde), after all his suffering and humiliations, is by far the wealthiest person at the table. In this, Voltaire has stretches the absurdity of the world (according to the expectations of the early moderns) to its ultimate endpoint, where even kings have been reduced of royal dignity, brought to the level of common men, even beggars.