ZeesKite: I asked my Rav tonight about the exact place where it says that it’s assur to take on unnecessary chumros. He told me that he is preparing a shiur on chumros vs. kulos for Shavuos, and didn’t want to spoil it for me. Bli neder, I will try to post the source after Shavuos, when I’ve heard the shiur.
He did confirm that it does say it.