Can Win 2000 machines, be connected / supported as a Secondary?
No, not with version 3 (the only version available).
For secondaries, is Win 7 minimum requirement?
WinXP but not fully supported (can't KVM to it for example).
Lastly, was there ever such Win2k support? Been a StarDock Object Desktop for 4 years or so. Might have "an old" KM Multiplicity v2 on a backup? If that can do it?
Perhaps but Win2k was a long time ago... You can try but if it does not work, I am afraid we would not be able to help with any issues.