There are plenty of good shrimp foods out there - your local fish store (called
LFS, you'll see people referring to them that way here), and also big box stores and online sell them. Check the label and make sure there's no copper in it. Copper kills crustaceans. I feed every 2-3 days. Check the package for amount recommendations.
Shrimp absolutely adore lotus
pods. In a small tank, one will suffice. They live in it like an apartment house and also nibble on it. Imagine eating your house bit by bit! Lotus
pods can be found online (I get mine from a really nice guy on Etsy).
They also adore Indian Almond Leaves (also called Catappa leaves). Online (I found them on ebay). These provide excellent tannins that will increase the health of everyone in there. They nibble at these too. The leaves turn the water brownish for awhile but that wears off, nothing to worry about.
They still need a bit of food, as well as these things they love to nibble.
A small tank can handle a mystery snail; more might tend to be crowded, especially if you have plants and a decor piece in there. Mysteries are so much fun to watch! They can be fed the same crustacean food/algae tab mix as shrimp. If you're buying to feed both shrimp and snails the package marked "sinking pellets" will be better for the snail's access, and the shrimp will go for it too.