Is it OK to connect some CDJ-900s with firmware Ver. 3.0 at the same time as other CDJ-900s with previous versions by PRO DJ LINK?

No.

The GUI (graphical user interface) and operation specifications differ, and connecting CDJs with different firmware versions may cause problems with DJ play.

If a CDJ with previous firmware version is connected, the message "A link to a player with a different version has been detected.

Update the player with the old firmware version." is displayed. Update all CDJs with previous firmware versions to Ver. 3.0. The latest version of the firmware can be downloaded from the Pioneer website.