Why does it say "El Derech ha'Goyim"?
Radak: This is like Es Derech ha'Goyim. Similar "Asaperah El Chok" (Tehilim 2:7) is like Asaperah Es Chok. "Va'Yirdefu El Midyan" (Shoftim 7:25) is like va'Yirdefu Es Midyan. Do not learn the way of the Goyim, since you are Mulim for a sign between Me and you. Guard My way, and not the way of the Goyim!
Is "ume'Osos ha'Shamayim Al Techasu" a command?
Rashi: No. If you do not go in the ways of the Goyim, you need not fear eclipses of the sun and luminaries. Malbim - this is because you are under Hashem's Hashgachah, and not under the Ma'arachah (arrangement of the Heavenly bodies). If you will learn the Goyim's ways, you will leave Hashgachah!
Radak: Yes. Do not say, we will serve the stars like the Goyim do, for they rule over the land and their signs are to benefit and harm. E.g. the sun has authority over the land regarding grain and Peros - "umi'Meged Tevu'os Shemesh" (Devarim 33:14). If we serve it, it will increase grain; if not, it will harm us and our grain will be decreased. And we will fear lest it shine intensely, which is a bad sign for the world. Do not fear it, or other Heavenly bodies, e.g. the stars. They are My servants and do My word. If you serve the Master, you will not need the servant. If you serve the servant, the Master will be angry at you, and send a curse in your grain - the opposite of your intent!
Malbim (9:25) Yes. Astrologers were saying that Nebuchadnetzar will rule over Arelim, but not over Mulim. Therefore, Hashem taught (9:25) that also Yisrael are Arlei Lev. We are warned not to believe in astrologers' words, for Yisrael are under Hashem's Hashgachah.
Why does it mention that the Goyim fear them?
Radak: Even though the Goyim fear them, you should not fear them, for if you serve Me, they have no authority to harm you, and their intense shining will not damage you. If they conduct normally, the world is sustained. Sometimes there is a loss in their trajectories; this is [a sign] for the other nations - "Asher Chalak Hashem Elokecha Osam l'Chol ha'Amim" (Devarim 4:19) - to benefit or harm them. "V'Eschem Lakach" (ibid., 20) - if you serve Him with a full heart, He will save you from all harm from the stars. Also Nochrim, their Avodah will not help them, for it is proper to serve the Master, and not the servant.
