Should one sit or stand for Kaddish? This seems to be a difference in practice between Ashkenazim and Sephardim, however, isn’t Kaddish considered a prayer like Kedusha or other prayer that requires 10 that you have to stand for? Also, what if one is in the middle of a sitting part of prayer such as Tachanun - should one temporarily stand for Kaddish if it is his regular custom to stand or can one continue sitting during this time?

Shalom, There are different customs about standing or sitting for Kaddish. The Rema writes in his additions to the Shulchan Aruch (O.H. 56, 1) "One should stand when they answer Kaddish". This is, as you wrote in your question, because it is counted as "words of holiness" ("dvar sh'bekedusha") which should all be stood up for (see the Rema ibid). However the Mishna Brurah there writes that there are those who say one does not have to stand for Kaddish, unless one was already standing. So, for example, the Kaddish after Hallel, when one is already standing because of Hallel, one should remain standing. This is how many Sephardim rule (see Yalkut Yosef, volume 1, p. 95). The Mishna Brurah rules that one should be strict and stand for every Kaddish. There are differing opinions also as to how much of Kaddish one needs to stand for - some say to remain standing for the whole kaddish, some until after "d'amrinan b'alma ve'imeroo amen", and some hold just until after responding "ye-hey shemay raba etc". If you are in the middle of tachanun, you must stop to answer Kaddish, but based on what we have written, one may remain seated if one so chooses. However, if the whole congregation is standing, one must stand with them so as not to separate from the congregation. (Rav Ovadyah Yosef shlitah rules that sephardim praying with ashkenazim must stand with the congregation). Blessings.