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!
You must daven the tachnun according to the nusach you normally daven. The kitzur shulchan oruch says that one cannot chop and change nuschaos as they are all weighted and each word is counted. If you would like to add vidui before tachnun then that's okay, but you should try not say sephardic tachnun when davening in ashkenaz l'moshol. Rav Moshe Feinstein was very strict on staying with one's nusach.