Someone who davens in a shtible that doesn't have a set , the chazzan chooses the nusach, as a congregant, what should one do. Harav Shlomo Zalman Auerbach said it is proper for a shtibel and a shull to have a set nusach but if it doesn't they should see what the majority of the congregants is. But it is much more common that the chazzan decides. Harav Shlomo Zalman Auerbach in Halichos Shlomo discusses someone who davens in a shull, and this would apply to a shtible too, what nusach should he daven. Typically one should but when one is ,אל תטוש תורת אמך – stick with his minhagim in a tzibur who has another nusach, one should not make it visible that he is doing something other than what the tzibur has chosen. If one is davening Psukei Dezimra out loud, he should adhere to the of the place. If he davens quietly, he can stay with his own minhag. The same with kedusha, which is a big chiddush, as there are many poskim who say that when it comes to davar shebikdusha, caused by the minyan, one has to adhere to the nusach of the minyan, i.e. if it's nusach Sfard one has to see nusach Sfard. In the footnote he says that the main parts of kedusha are the same, there are only differences in additional piyutim that were added on, Keser yitnu lecha, mimkomcha etc. One can say his own nusach,

but then one should be sure to say it quietly. At the end of the davening there is a different order, for e.g. says right after and other nuschaot for e.g. Sfard says Aleinu at the end after and Ein Ke'elokeinu, in such a situation one can say what he wants but make sure it's not visible. It is more important at the end of the davening to stick with the minhag of the shull. This applies also to , one should not stand when they are sitting and not sit while they're standing. The key is not to do something which is viewed by others, an action or words that can be heard.