DDJ SZ with Rekordbox and 2000 nxs 2

Not "send" tracks to the players, but rather use the CDJs as controllers for rekordbox, yes. As you're using an SZ and CDJ-2000NXS2's, neither of which are rekordbox 6 hardware-unlock devices, you would need to either have a license for rekordbox 5, or a subscription for rekordbox 6.