shouldnt the command center be higher priority than an ion cannon or mass driver? if the planet is isolated, killing the command center removes supply from most defences, prevents defences being rebuilt and prevents reinforcements from spaceports.
agree w/ AI units should ignore ion cannons of lower mark than themselves.
I dont know much about how this sort of thing is done, but how about only allowing the AI units to attack ion cannons/mass drivers if it can simultanously order x number of units to do so (ie, the minimal number realistically needed to destroy it in timely fashion)