well, what kind of plants lol. isolate them so they can regrow so u dont waste your money. and perhaps feed your fish and then see if they leave them alone?
Well these fish love algae so I suppose its possible but I've never had that problem with any livebearer at all. Maybe buy some algae disks and throw one in there? They'll peck on it and be occupied with it for a while?
Nor have I, platy and molly jaws aren't all that big or strong so I can't see them shredding plants, it's possible that the plants are posing some leaves from the shock of moving and the fish are nibbling at the dying leaves, I've kept platies in planted tanks for a long time and never had a problem. Then again, maybe I'm wrong and you have some GIANT platies!!