Who are "Rashei Livyasan"?
Rashi: Pharaoh is called so - "Yifkod Hashem b'Charbo ha'Kashah... Al Livyasan" (Yechezkel 27:1). Radak - Livyasan is the great Tanin - "Al Livyasan Nachash Bari'ach (ibid.)! It says "Rashei", for also his officers and Giborim [were crushed].
Malbim: They are the officers and leaders of Egypt.
How were they food "l'Am l'Tziyim"?
Rashi: You gave their money to Am Yisrael to consume it. "L'Tziyim" are to Your groups and Tzeva'os (legions) that You took out. The Targum of "v'Tzim mi'Yad Kitim" (Bamidbar 24:24) is v'Siyan.
Radak #1, Malbim: Yisrael took the spoils of Keri'as Yam Suf (the Egyptians' wealth); they are a nation going l'Tziyim (to Midbaros) - Midbar Eisam, Midbar Suf, Midbar Paran, Midbar Kadesh, Midbar Sin, Midbar Mo'av.
Radak #2: Desert animals and birds 1 .
I.e. they ate the Egyptians washed up onto the shore. It is not clear why they are called "Am". Mechilta d'R. Yishmael (Bo, Piischa 13) learns from "Tivla'emo Aretz" (Shemos 15:12) that the land buried (swallowed) the Egyptians. Perhaps Radak holds that the commoners were buried, but not the officers and Giborim (refer to 74:14:1:1). (PF)
