Long Arm Shrimp is a generic term given to many species of shrimp, most of them are Macrobrachium species. These shrimp range from the Pearl Shrimp, a small peaceful shrimp, to the Macrobrachium rosenbergi that can get a few feet in length.
So answering a question about Long Arm Shrimp is next to impossible. Will they eat plants? YES some of them will, but then again SOME of them will not. Some of them will eat every fish in your aquarium; some of them will not bother another living animal! So, to answer your question correctly, we would have to know the species!
Now lets talk about the Ramshorn snail debate. There are 2 types of Ramshorn snails that are commonly found in the aquarium hobby. One comes from Europe (Planorbis corneus) this Ramshorn snail will not eat live plants at all. Then there is the Ramshorn that comes from the Central America Region and they will devour plants. These snails tend to be a bit larger and are sometimes referred to Giant Ramshorn snails (Marisa cornuarietis)! So odds are you will get the more common European Ramshorn Snail, but there is a chance you will get some that will mow down your plants as well!