I don't know if these claims of amphipods eating zoa's are true or not, but I know for a fact I have a ton of amphipods in my tank and I just recently started collecting zoa's. So far my zoanthids are doing OK, but it is kind of early to tell if they are going to thrive or not. However, since my tank is mainly an sps tank I have very high flow and the zoa's don't seem to like this very much so their well being might be affected by this also.
If you are looking for a good natural predator of amphipods and you don't want to go with a wrasse try a pipefish. I have a banded pipefish in my tank and he is an amphipod vaccum. Now since their mouths are so small they can only eat the smaller amphipods, however they do an amazing job at keeping the population in check and preventing the huge swarms of giant amphipods that everyone is talking about. Plus, they are the coolest fish I've ever kept and very interesting to watch hunt. They are completely reef safe, they are basically oblivious to anything else in the tank other than small crustaceans and none of my other fish seem to mind it. I wouldn't get one if you already have a wrasse or any other very agressive fish. I have 3 different tangs and two clownfish and everyone gets along just fine.