Shrimp are generally more sensitive then fish, so a filter is a must. Depending on the shrimp, a 2g is suitable. You could do something hardy like cherry shrimp, or one of their hardy color variants (discluding a couple that are extremely inbred and sensitive), or sulawesi shrimp would be a good fit as they are extremely small, and very hardy as well. I would steer clear of amano as they are quite large, crystal red because they are sensitive as well, most of the dwarf shrimp really just get a little to large for such a small sized tank, so do a little research on the individual type before you get it.
As ninjouzata said, you can buy or make a cheap sponge filter, that is the best for shrimp.