A freshwater dip on zoanthids is not going to kill them. Honestly, Coral Revive, etc is better, but freshwater dips work just fine for some corals, especially Zoanthids.
As for the Nudibranchs, just really keep an eye on any polyps in your tank (zoanthids, cloves, palys). If you see them, dip. They suck to get rid of and you do have to stay on top of them, BUT, it's not a death sentence, you can beat them.
Do yourself a favor and a) find another store and b) dip corals BEFORE they go in your tank.