You don't need a filter in a shrimp tank. You could easily get away with some guppy grass, some Amazon frogbit, and maybe some anacharis or moss of some type.
Any type of neocaridina will be fine in that tank. This includes red cherry shrimp, yellow shrimp, blue pear shrimp, snowball shrimp, etc. Do not mix them or else they will interbreed and produce offspring that are more like the wild type.
You could do caridina species as well, but these are typically more expensive and with such a small volume of water it can be hard to keep levels maintained properly, so a pH swing, ammonia spike, etc. could kill off all of your stock of caridina species.
Ghost shrimp would also work, though I don't recommend housing them with other shrimp. You might also check into Amano shrimp. They would go great in a planted tank, as would many of the neocaridina species shrimp mentioned above.
