A planet has a multiplex tracking level of 10 by default. Adding multiplex to your WPs will not change this. In fact, it's not available in the design screen for WPs by default. There's actually a "fix" in the history file where he says he stopped the AI from putting multiplex and ECM on WPs. Apparently ECM on a WP doesn't have any effect either. So all you need is the computer core and combat sensors. Then stuff it with weapons and armor and shields.
