If one is davening at a minyan where they don't say tachanun, for reasons that the individual does not feel tachanun should be skipped for, should they say tachanun or skip it? If they should say it, should they leave the room to do so or should they say it in place? Is there any way to permit the person to say it later?
I asked this question to Reb Dovid Feinstein with respect to a Chassidish shul that skips Tachanun on various days when I don't, and he said I could ignore that custom and say Tachanun there.
If the reason the tzibbur is not saying tachanun is because of a minhag not brought in halacha, such as not saying tachanun when there is a yahrtziet of a tzaddik, a person that wants to say tachanun may do so inside the shul.
If he is in a minyan that regularly doesn’t say tachanun by mincha, he should do like them. The reason for this is that it is considered a minhag done with a halachic basis, since we don’t say tachanun at night and there are places that used to daven close to night time, and in order not to say tachanun at night they don’t say it at all by mincha.