What was the significance of the very first scene?

You mean the one in which the knight of Milletail pees on the old guy?

Well the old guy is the Comte de Blayac, who used to be a very fashionable man at court because he was apparently very witty. Milletail once fell during a ball and Blayac created a nickname for him, "Le Marquis de Patatras" ("patatras" = the noise when someone falls heavily to the floor in French). This nickname ruined Milletail's reputation (and therefore his life) and he had to leave for America. Now he's back and Blayac has become old and a helpless cripple, so he gets his revenge : he pees on him and then makes the maid believe that the man pissed his own pants (since he can't talk anymore he can't defend himself).

SPOILERS! The first scene set up that this was a time in which one man (Blayak, the old man) could ruin another man's life (Milletail) by publicly ridiculing him, and that the result could be so severe as to cause him leave the country. (I love it that Milletail does return for revenge and literally takes the piss on the old man!). But more important, this also sets up the dramatic impact of Grégoire's fall when he is tripped at the dance toward the end of the movie. The viewer imagines the ridicule to be bestowed upon Grégoire and that his plans to save his hometown are doomed. Grégoire's revenge may be more genteel than Milletail's, but Madame de Blayac certainly deserved a golden shower.

The guy in the beginning who pulls out his peen and pees on the old guy, is the same guy at the end who calls Ponceludon "Marquis de Antipodes" (sp?) and who feels ashamed / regret for doing so, after Ponceludon asks him to take off his masks - because he has been through the same thing.

This humanizes the perpetrators and shows that often victim and perpetrator are one and the same, just a different time and context. The guy in the beginning is no different from Ponceludon except for having taken revenge. In fact he is no different from Ponceludon at all - Ponceludon doesn't change the system he just works it. Likewise with the other guy - he may not necessarily even know why Ponceludon is being set up, he's just going with what everyone is doing.