For that sized tank, there are only 2 hosting anemones that I would even consider -- an BTA (( E. Quadricolor )) and an H. malu. But, that is only when the tank is ready, and you for that matter. With smaller tanks, water quality can change very quickly -- which isn't a good thing for anemones.
BTA's are easier to come by, and should be able to found as a clone. H. malu are harder to come by, and more difficult to care for, but would stay smaller then other hosting anemones -- which would get too big.