In my 1.5 gallon tank, I have installed a Whisper filter system. I was thinking ...
It is possible to have a snail and/or an African dwarf frog in a 1.5 gallon tank with a filter. Just be careful not to overload the tank with fish or other creatures. A small tank like that is limited to only a few pets. It also requires regular water changes. We recommend a weekly water change of about 10% of the water. Also, regular rinsing of the filter pad in your Whisper filter to keep it free of debris.
Both snails and African dwarf frogs will eat flake and frozen fish food. You need to be sure there is some available to them on the bottom, but don't overfeed! Uneaten food pollutes the water and can kill your fish.
PetSmart offers free water testing, and you can also pick up free care guides on the pets you mentioned, and on setting up and maintaining an aquarium. These guides have more information on the water testing and water changes, and how to feed your pets.