DSC zone expander connects to the four wire keybus. From the DSC manual:

-Minimum 22 AWG wire, maximum 18 AWG (2-wire twisted preferred
-Do NOT use shielded wire
-Modules can be home run, connected in series or can be T-tapped provided
that the maximum wire distance from the control panel to any
module does not exceed 1,000 feet (305m)

I would get proper gauge 4-wire instead of Cat-5. No problem powering from the main board.

Definately put it on a separate partition as it simplifies arming, etc.