Davening Mincha after accepting Tosefes Shabbos:
One may Daven Mincha even after accepting Tosefes Shabbos, and so is the widespread custom. However, once one has accepted Itzumo Shel Yom [such as Davening Maariv] he may no longer Daven Mincha.
 Implication of Admur Seder Hachnasas Shabbos who allows Davening Mincha even after sunset; P”M 263 A”A 28 that if he did not say Barchu, he may still Daven Micnha; Mishmeres Shalom 26/23; Eretz Tzevi 1:60; Minchas Yitzchak 9:20; Piskeiy Teshuvos 263:41; See Admur 393:2 regarding the difference between accepting Tosefes Shabbos and Itzumo Shel Yom
Other opinions: Some write one is to be particular to Daven Mincha before accepting TYosefes Shabbos. [SSH”K 46:5]
 The reason: As accepting Tosefes Shabbos is only regarding the Mitzvah of Melacha and not regarding
 Admur 263:23; Michaber 263:15; Mordechai 296; Hagahos Maimanis 5 Reish