Can someone start rosh hashana early after plag hamincha on the first day? This year shboos is on the mozei rosh hashana: Can shabbos be made eary after plag hamincha by making kiddush, and davening mariv later ?
It is fine to accept in Rosh Hashanah early (as one does for regular Shabbos). There are no special mitzvos that must be performed on Rosh Hashanah night, and the concern for performing a Yom Tov mitzvah before nightfall, which applies to certain mitzvos, does not apply.
It is likewise fine to accept Shabbos early by making kiddush after pelag ha-minchah on Erev Shabbos (Shuchan Aruch Harav 527:8).
In general, the Vilna Gaon (in Maaseh Rav) objects to making kiddush before davening Maariv, but it is clear that many rishonim uphold this as an option for bringing in Shabbos early.