Pods really don't attack healthy zoas. How do you acclimate your new corals and polyps? Do you test the salinity between the bag water and your tank? Are your tank params OK? Reason I ask, is that it sounds as if your new frags could have been either dying or severely stressed. The fact that the 'pods are not attacking any of the other zoas is the main thing that leads me to believe that this is what happened. 'Pods are scavengers, and very helpful to your tank. What you are describing sounds like an amphipod. Do a search on google and you should find a picture fairly quickly...LMK if this is what you have?
Oh - FYI, there *is* a type of snail that will eat zoas. I'll try to find you the thread that describes it...found the info on Wet Web Media. ;)

Wendy