No the snails will not eat pleco poop, and even if they did they'd just convert it to snail poop anyway.
The only way to get poop and its by-products out of your tank is through water changes and vaccuming.
On the subject of Dwaf Puffers eating snails, pica_nuttalli is correct, the puffers will eat what they can get to and leave the rest to rot at which point you need to remove the shells before they pollute the water.
In my experience Dwarfs can eat snails way faster than the snails can reproduce. Snail hatchlings are relished by the puffers who will crunch them up.
Dwarfs will also eat the eggs.