fan/filter shrimp will not eat your fish, they don't have claws.
filter shrimp eat waterborne algae and infusoria. They generally like to sit by the outlet of the filter wih heir fans open until they have collected enough and then they will eat what is in that fan. This is why they aren't really suitable for new tanks which have no infusoria. However they will eat anything that is small enough to become trapped in their fans, such as liquifry (this is often used as a supplement), fine powdered flake, and small frozen or live foods such as daphnia or BBS.
You will know if your shrimp is not getting enough to eat because it will run around the bottom of the tank runnng its fans over the substrate in an attempt to get more food. If you see this, feed more.
As for sand, I don't believe it matters. And sand in the water won't hurt the gills on any fish. There are many species who routinely blow sand through their gills.