I am Ashkenaz, but regularly attend a Syrian Sepharadi Shul.
Both Mincha and Maariv are scheduled after Plag Ha’Mincha, but before Sh’kiya.
Can I continue to daven maariv at this Shul before shkiya l’hatchila?
Is davening maariv alone after Tseit Hakohavim a better alternative?
The Magen Avraham (233:7) implies that one should daven in this case with the congregation, and not wait to daven maariv alone after nightfall.
This is the ruling given by the Aruch Ha-Shulchan (235), and by others (see Shut Me’at Mayim no. 67), the Aruch Ha-Shulchan explaining that the great virtue of tefillah be-tzibbur is not deferred even under these circumstances.
However, there is room to defer the proof from his statement, and several poskim write that under circumstances where both mincha and maariv are davened after pelag ha-mincha, one should not daven maariv with the congregation, but rather wait to daven later, alone.
This is the ruling given by Zichron Yehudah (p. 25, in the name of Maharam Ash), and this is clearly implied by the Mishnah Berurah (see Shaar Ha-Tzion 235:16).
Therefore, both options are valid, and a person has the right to choose between them.
In places where the custom is to daven mincha and maariv together, there might be an issue of yuhara in showing that one does not daven with them, and one should be careful of this (see the above-mentioned Shut Me’at Mayim, who writes that somebody in a shul who plans to daven later (with a minyan) should pretend that he is davening with them).