Since the first stone contained the names of six tribes, it would have sufficed to write "ve'es Sh'mos ha'Nosarim". Why does the Pasuk add the word "Shishah"?
Oznayim la'Torah: To preclude including both 'Efrayim' and 'Menasheh' on the second stone 1
What exactly, is the meaning of "ke'Soldosam"?
Rashi and Targum Yonasan (on Pasuk 16): It means literally in the order that they were born 1 - Reuven, Shimon, Levi; Yehudah, Dan, and Naftoli on one, and Gad, Asher, Yisachar, Zevulun, Yosef and Binyamin 2 on the other. 3
Ba'al ha'Turim: Refer to 28:16:5:1*.
Da'as Zekenim: Really, k'Soldosam refers only to what is written just before it (the six on the latter stone), and not to those on the first stone. 4
Da'as Zekenim #2: One opinion in Sotah 36a says that k'Soldosam teaches only about the spelling of their names
See Sifsei Chachamim.
Written Malei, with a second 'Yud' (as it was spelt when describing his birth (in Bereishis 35:11) so that there should be twenty-five letters on each stone (Rashi).
The problem with this explanation is that it clashes with what Rashi himself writes in Seifer Shoftim, 18:28, where, based on the Pasuk, he explains that the stone Leshem corresponded to the tribe of Dan, which is not the case according to Rashi here (See Chizkuni).
As the Gemara specifically states in Sotah, 36a.
See Oznayim la'Torah.
QUESTIONS ON RASHI
Rashi writes that they were in the order that they were born. In Sotah 36a, all hold that Yehudah was first!
Refer to 28:10:1:3,4.