1)
Who are these slaves?
1.
Radak: This is like it says "v'Amdu Zarim v'Ra'u Tzonechem u'Vnei Nechar Ikareichem v'Chormeichem" (Yeshayah 61:5). Also them, since they will be in Eretz Yisrael and serve Yisrael, they will have Ru'ach of De'ah and Sechel.
2)
Why does it say "Eshpoch Es Ruchi"?
1.
Radak: There will be abundant Ru'ach, like something spilled - "v'Shafachti Al Beis David v'Al Yoshevei Yerushalayim Ru'ach Chen v'Sachanunim" (Zecharyah 12:10).
2.
Malbim: Even though they will not have true dreams, and all the more so, not higher levels of prophecy, they will have "Ru'ach Etzah u'Gevurah... v'Yir'as Hashem" (Yeshayah 11:2) and similar attributes of the Nefesh.
