Is one to recite Nakdishach if he is Davening by a congregation that says Nikadesh?
If one is Davening by an Ashkenazi Minyan who recites Nekadesh instead of Nakdishach, he is to recite the Nussach of the congregation, which is Nikadesh. The same applies during Musaf, that he is to recite Nakdishach in place of Keser. If however there is a Minyan of people saying the Nussach of Nakdishach/Keser, then he may recite it.
 Igros Moshe 2/23; Minchas Yitzchak 7/5; See Piskeiy Teshuvos 68 footnote 29; See however Maharam Brisk 2/28; Piskeiy Dinim Tzemach Tzedek 236 regarding that Davening a different Nussach than the congregation does not involve Lo Sisgodedu [He however does not discuss the scenario of Nakdishach] Maaseh Rav of Rav Nassan Adler that Davened Nussach Arizal in his congregation of Nusach Ashkenaz, brought in Chasam Sofer O.C. 16; Sefer Haminhagim p. 5 [English] footnote 16; Igros Kodesh 10/208; 14/391 19/4 [Published in Shulchan Menachem 1/171-174]
 The reason: As even if there is no issue of Lo Sisgodedu, one requires a Minyan when saying Nakdishach, and if the congregation is saying a different Nussach then he does not have a Minyan for his recital. [ibid; See Admur 125/1]
 The reason: As the entire issue with saying Nakdishach is that he cannot say a Davar Shebekudhsa without a Minyan. Thus, if there are at least ten people saying it, then he has a Minyan available. Thus, by a mixed Minyan in which the Sefaradim and Chassidim all say Nakdishach, one may also do so even though the Chazan Davens Nussach Ashkenaz.