Shir Hayichud

In some Kehillos Shir Hayichud was said every day, and in some it wasn't, and in some there were individuals who had the Minhag to say it (together) every day. Those who have a Minhag to say it have to continue saying it, but a Kehillo that doesn't have the Minhag to say it, doesn't have to start saying it.
Most Kehillos said Shir Hayichud on Shabbos, because on Shabbos one has more time, and in addition the Shir Hayichud for Shabbos is short.
Shir Hayichud does not require an open Ark, only Shir Hakovod (Ani'im Zmiros) does. It is not recommended to say Shir Hayichud or Shir Hakovod without a Minyan, since some Poskim say one isn't permitted to do so.

FRBSH

In KAYJ we say Shir Hayichud on Shabbos and on Yom Tov (unless we are very close to Chatzos), but not on Chol Hamo'ed and weekdays.

Michael

I asked Moreinu Harav Gans Shlito, and he said that if you did the Hatoras Nedorim and Mesiras Modo'oh on Erev Rosh Hashono, you can rely on it to stop saying Shir Hayichud. You shouldn't continue saying it, since you aren't saying it with a congregation.

Rav Gans told me that generally one cannot rely on the Hatoras Nedorim for whatever he said Beloshon Neder, but for good deeds done 3 times etc., he may.

Michael