Question: I usually daven nusach ashkenaz, though sometimes daven in a nusach sefard shul, and am confused what I should do when the davening is different.
Answer: There are a few minor differences between different nuschaos. While it is important that one follows the nusach of one’s father or community, the Gemara (Yevamos 13b) writes that the prohibition of lo sisgodedu (Devarim 14:1) applies to practicing different customs to each other. Whether this applies here is a matter of debate.
The Netziv (Meshiv Davar 1:17:7) and R’ Yitzchak Yaakov Weiss (Minchas Yitzchak 7:5) write that when davening the silent amida, one should daven according to one’s own nusach. When davening anything out loud, however, one should adapt to the shul’s nusach. Unlike the amida that one can daven alone, one can only say kedusha, for example, with a minyan. Thus, in a sefard shul one should say nakdishach and kesser in kedusha rather than nekadesh and naritzach, etc.
R’ Ovadia Yosef (Yabia Omer OC 6:10:4) argues that one saying the introductory phrases such as naritzach out loud does not transgress lo sisgodedu as they are joining in with everybody else for the main pesukim. Therefore, everyone may daven according to their own nusach even for tefillos said out loud.
Nonetheless, R’ Moshe Feinstein (Igros Moshe OC 2:23; 2:104; 4:34) writes that one must join in with everyone else throughout the entire kedusha. As one is supposed to recite kedusha along with the chazzan, this applies even if one is saying the words quietly. One may recite one’s amida in one’s own nussach but one should try to say the rest of davening according to the nusach of the shul.
In conclusion, one must join in with the shul’s nusach for any parts of davening that are recited aloud. Ideally, one should adapt to the shul’s nusach for all of the davening except the silent amida.