Davening Somewhere with a Different Minhag
13 Middos, Vidui
Minhag not to say. Someone with the minhag not to say the 13 Middos and Vidui who is davening in a shul that says them should not deviate from their minhag; he should say them with the tzibbur (שו"ת אגרות משה או"ח ח"ג סי' פ"ט ).
Minhag to say. Someone with the minhag to say them who is davening in a shul that does not say them should say Vidui in an inconspicuous way without tapping the heart (שו"ת אג"מ או"ח ח"ד סי' לד, שו"ת אור לציון ח"ס פ"ט תשובה א, הגרי"י קנייבסקי ארחות רבינו ח"א עמ ' סח). He should not say the 13 Middos, as they may not be said without a minyan (אג"מ שם ). Some say them with their trop (אור לציון שם ).
Putting the Face Down
An Ashkenazi davening with an Eidot Mizrach tzibbur, where they do not put the face down, should follow their minhag and not put his face down (הגרשז"א, ס' ועלהו לא יבול סי' קל"א ).
Changing the Order
On Monday and Thursday, Nusach Sefard first puts the face down, then says והוא רחום (שערי תשובה סק"א ). Nusach Ashkenaz first says והוא רחום, then puts the face down.
If one is in a shul that says Tachanun before והוא רחום, e.g., a Nusach Sefard shul, and that is not his minhag, or vice versa, there is no issue of לא תתגודדו since today there are differing minhagim everywhere (הגרשז"א, אשי ישראל פכ"ה הע' כ"א, הגריש"א תפילה כהלכתה פ"ד הע' כ "ו ). Additionally, people daven at different speeds, so he is not clearly acting differently than the minhag of the place.
