1)
What should they be Metaher?
1.
Ibn Ezra: Themselves; it should say Mistaharim.
2)
Should the Leviyim guard the gates specifically on Shabbos?
1.
Malbim: Yes. Due to Kedushas Shabbos, the guards should be Leviyim, who are more Tahor and more careful about any evil [than Yisrael]. They will oversee Kedushas Shabbos more.
