1)
Why does it say "Mishmeres Le'umas"?
1.
'Rashi', Radak: It means Mishmeres Le'umas Mishmeres. We find like this above (17:5) "va'Ehyeh me'Ohel El Ohel umi'Mishkan" - it means umi'Mishkan El Mishkan.
2.
Refer to 25:8:2:2.
2)
What do we learn from "ka'Katan ka'Gadol Mevin Im Talmid"?
1.
'Rashi': Mevin is the Rebbi. All of them were the same. Whomever the lottery will decide is first, he will be first, even if he is small.
2.
Malbim: It means Mishmeres Le'umas Mevin Im Talmid ka'Katan ka'Gadol - they arranged that every Mishmar will have those who know to sing, and Talmidim, who do not know so well; they learn from those who know. The greater and smaller were the same in the lottery (refer to 24:31:3:1).
