Most of your plants are slow growing, so not great N sponges.
I would run a sponge filter with a power head. Set the power head below the water surface so there is gentle ripples at the surface. Then you can get more flow through the sponge without blowing away your CO2.
Your bio load is not a problem for a sponge filter, if you are running it right.
While your tank is still cycling I would vacuum away the poop and any fallen food, and feed less.
Fish food is the main source of nitrogen in the aquarium. If your fish will eat low protein foods that would be better. Otos and snails will eat kitchen produce- blanched vegies. Neon Tetras are not very likely to eat vegetables.
A 20 is not big enough for Mystery or Apple snails. I would return them and try one of the smaller Nerites, or perhaps a few of that yellow and black striped snail with the 'horns'.