Following up to this question. If one holds Rabbeinu Tam always but happens to daven in a minyan on Yom Kippur that doesn’t hold this zman, can she continue to daven Maariv there or should she leave and go to another shul that does hold Rabbeinu Tam or just daven Maariv later b’yechidus? If one doesn’t have anyone else to make havdala for her, can she hear earlier and be yotze from someone who doesn’t hold Rabbein Tam and wait to eat later?
If one usually holds the zman of Rabeinu Tam can one daven with a minyan maariv motzai Yom Kippur and hear havdala before Rabbeinu Tam
It depends why the person keeps Rabbeinu Tam. If it is because you hold that this is the halacha, then it should be kept even for Yom Kippur,however if it is because for the chumra of shabbos you keep it but really you hold that it is nighttime before that, then on Yom Kippur you can be lenient because of the great need.
It is good that you clarified he question because I though it was because you want to eat earlier. A person that keeps a later zman is allowed to daven maariv and even make havdalah before the zman that he keeps, and he should be careful not to do actual melacha until the time that he is makpid about. This would apply not only to yom kippur, but to every shabbos too. Regarding Yom Kippur he should also be careful not to eat until the zman that he usually keeps.
Shulchan Aruch O:CH 293-3.