A 20 gallon tank is small for most any anemone. It is tough to keep the water parameters pristine enough for them.
Also, what kind of lighting do you have? You would need IT LEAST 100-120 watts of PC, VHO or MH lighting.
As far as too soon, I would be hesitant to put an anemone in any tank less than 6 months old. give your LR and sand time to mature a bit.