Granted i'm ignorant of supergirl's stories but it that usually a theme of her stories? Not to say they won't tackle the immigration angle but it seems, to me at least, that Supergirl's social messaging tends to mean more on girl power side. So it seemed to me like it would make more sense to tackle a feminism angle more so than immigration.
As for Cyborg I'm less sure if they will use him. I only figure they're still using John because he was one of the names they announced first and considering he's one if their biggest names. Also the fact that the character has his roots in social activism, it would be weird not to use him.