I have heard some people on this site mention they found their ancistrus sp. eating plants, but of the few different species i have kept, none have ever touched plants. This simply is because some won't eat them, and some will, depending on species. Of all the species, i don't have a list of which will and which won't, and obviously individuals are different. Just feed them mainly on vegetable matter and then hope that you got a plec that is happy to live around the plants, and not see them as food.