If one davens mincha before plag, is it l’chatchila to daven maariv after Shkiah?
If one davens mincha before plag, it is permitted, according to most authorities, to daven maariv early — meaning after plag, even before shkiah.
However, the best time to daven maariv, even when one davens mincha before plag, is after tzeis hakochavim.
There is an advantage of davening after shkiah, even before tzeis, because some maintain that the time for maariv according to the Shulchan Aruch is from shkiah (see Or Letzion 2:15:6). The Mishnah Berurah (233:9), however, and most authorities, maintain that the time for maariv is from tzeis hakochavim.
When davening before tzeis, the Shulchan Aruch (235:1) writes that one should recite the blessings of the shema and the shema itself, with the congregation, and return to recite shema again after tzeis. This is the common custom. However, others (including the Mishnah Berurah, see 235:12, and Iggros Moshe Orach Chaim 2:60, and see Piskei Teshuvos 235:3) write that it is better to recite the blessings of the shema later, after tzeis, together with the shema.
According to the Vilna Gaon, it is better to daven biychidus after tzeis than to daven with the tzibbur before tzeis.