Why does the Torah see fit to mention that Reuven was the firstborn of Yisrael?
Da'as Zekenim, Rosh: Since Yehudah traveled first, the Torah explains why Reuven is listed first. 3
Seforno: Even though b'Dinei Adam, he lost it (Perhaps that is why the word "Bechor" is missing a 'Vav' - EC).
Even though the Torah did not yet mention the Degalim, we find that the Shevatim are listed according to the Degalim, e.g. Gad is after Shimon. (PF)
Why does the Torah write "l'Gulgelosam" only regarding Reuven and Shimon, but not regarding the other tribes?
Da'as Zekeinim, Rosh: Because Ya'akov rebuked them 1 when he blessed his other sons, and they needed a Kaparah - and in connection with the half-Shekalim, which came as a Kaparah, the Torah wrote "la'Gulgoles" (Shemos 38:26).
Riva: They needed a Kaparah