Ledavid Hashem Ori Part 2:
If one is praying in a Shul that says Ledavid after Aleinu what is he to do?
He is to follow the congregation. Thus he is to say Aleinu with the congregation and then say with them Ledavid. However there are Poskim which rule one may follow his custom and say Ledavid before Aleinu.
When is the Kaddish Yasom to be said, before or after saying Ledavid?
The Chabad custom by Shacharis is to recite the Kaddish only after saying both Shir Shel Yom and Ledavid. This Kaddish counts for both Ledavid and Shir Shel Yom. [On Rosh Chodesh one says Shir Shel Yom, Barchi Nafshi, Ledavid and only then is the Kaddish recited.] Likewise by Mincha our custom is to recite Kaddish only after Ledavid and Aleinu and this Kaddish counts for both Aleinu and Ledavid.
Segulos involved in saying the above psalm:
One who recites the psalm of Ledavid from Rosh Chodesh Elul until Simchas Torah is able to tear any evil decree which has been placed upon him and is acquitted from judgment.
The recital of Ledavid is a Segula to banish all the evil forces and prosecuting angels. During the days of Elul that the thirteen attributes of mercy are revealed it is possible also for forces of evil to receive nurture and therefore we recite this Psalm to banish them from nurturing from holiness.
 Igros Kodesh 19/430
 Vetzaruch Iyun as what is the reason for this ruling? Perhaps it is done in order to say Aleinu with congregation. [See Elya Raba 237/4; Machatzis Hashekel 65/6; Ketzos Hashulchan 13/6] Vetzaruch Iyun if based on this one can deduce that whenever one is Davening in a Minyan and they reach Aleinu before him, one is to stop and say Aleinu with them, even if he is in the midst of Davening [in an area that he may stop].
 Divreiy Moshe 1/35 [See there for an analysis on the Issur of Lo Sisgodedu. No mention is made regarding saying Aleinu with the Minyan.]
 Shaar Hakolel 11/29 in name of Reb Hillel Paritcher; Sefer Haminhagim p. 112 [English]; Toras Menachem 25/230 [brought in Shulchan Menachem 3/73]
The reason: As it is our custom to diminish in the amount of Kaddeishim we recite, and to only recite the Kaddeishim that were instituted by the previous Sages. [ibid; See Beir Heiytiv 55/1]
Other Opinions: Some have the custom to recite two Kaddeishim of Kaddish Yasom; one after Shir Shel Yom and one after Ledavid. Likewise by Mincha they recite one after Ledavid and another after Aleinu. [See Siddur Beis Yaavetz-additions of the editor]
 See Divrei Yosef p. 166-169
 Sheim Tov Katan brought in Piskeiy Teshuvos 581/11
 Siddur of Reb Shabsi brought in Shaar Hakolel 11/28
 See Shaar Hakolel 11/28