According to the pinned thread in the invert forum on snails, their types, and IDing them:
1. MTS eat algae, leftover fish food, and dead plant material. I have forum friends that swear by MTS. I have a seeding of trumpets in all my betta tanks. I am not finding empty shells. The betta check them out and then leave them alone.
2. Pomacea bridgesii, Apple Snail, can starve in a planted tank without food. They have a trap door that protects them. They eat algae, fish food, boiled veggies.... They get up to 4" in a wide variety of colors.
There are different species of pomacea. Others that DO eat plants.
My supplier laughed when I asked if my betta could hurt them. "Ohh!! yeah we are terrified!

"
I have put some in the goldie tank with a 6" common. If he could eat it, he would.
From what I've heard, some aquatic life will mow down a forrest, but most nibble on the dead and dying leaves--a good thing.