What is the Shichor?
This is like Targum Yonasan and Yerushalmi on that verse. Ibn Ezra, R. Bechayei, Ha'Emek Davar (all on that verse), Kaftor v'Ferach 11 citing Rav Sadya Gaon disagree. Refer to Bamidbar 34:5:1:2 and the notes there.
What is the meaning of "Gevul Ekron Tzafonah"?
Rashi: Ekron is further north than the other cities.
What do we learn from "la'Kena'ani Techashev"?
Rashi: It is from Eretz Yisrael that Hashem gave to Avraham.
What are Sarnei?
Radak: They are nobles.
Where are the Sarnei Pelishtim?
Rashi: They are at the end of the western border of Eretz Yisrael, by the sea.
The verse says that there are five, but it lists six!
Rashi, citing R. Yochanan (Chulin 60b): Five were esteemed; the Avim were not esteemed.
Rashi: The Avim are not counted, for they are not Pelishtim. Rav (Chulin 60b)
Tosfos (60b): This refers to Teiman ben Elifaz.