Lighting yizkor and yahrzeit candles for rebbeim

Question

I have heard of a custom in Chabad to light yahrzeit and yizkor candles for the rebbeim. But I literally don't know anything about it except that some in Chabad do it. Can anyone tell me:

1. How common is this practice inside/outside Chabad?
2. Who inside/outside Chabad practices it?
3. How do they practice it? Are additional yizkor candles lit beyond that/those for loved ones? Are yahrzeit candles lit for all 7 (9?) rebbeim of Chabad?
4. Generally, how common is the practice of lighting yahrzeit/yizkor candles for non-family, i.e. tzaddikim?

Answer 1

The source of this custom is a letter (From Rosh Chodesh Shvat 5711) of the Lubavitcher Rebbe, where he writes what one should do for the first Yahrtzeit (10 Shevat 5711) of the Previous Lubavitcher Rebbe:

להדליק נר שידלק כל המעת לעת. אם אפשר בקל - נר של שעוה.

A yahrtzeit candle should be lit that will burn for the entire twenty-four hours. If possible, the candle should be of beeswax.

Generally, the custom is to light it for the Previous Lubavitcher Rebbe (Rabbi Yosef Yitzchok Shneersohn, on the 10th of Shvat) since the current Lubavitcher Rebbe said to, and for the current(Rabbi Menachem Mendel Schneerson, on the 3rd of Tammuz).

Iit could be that the older Chassidim who were Chassidim of the fifth Lubavitcher Rebbe still lit a candle on his Yahrtzeit, but I don't think that any are still alive at this point (he passed away on the second of Nissan 1920).