halacha - Which Tachanun should I say when davening in a minyan with a different Nusach?

I daven Nusach Ashkenaz but I often daven mincha in minyanim with a different nuasach. Should I say my ashekanzi tachanun or is there a minhag ha'mokom? Moreover I have noticed sefardim davening their own tachanun in my ashekanzi minyan. If I can get Halachic sources that would be great!