A 20 gallon might be ok for awhile but chances are one will turn to female and get to big and bossy for the tank.
One thing that is very different from freshwater is topping off it is very important especially on a smaller tank.
You will want live rock it is important for the biological filter. Coral is something you might want to try in the future.
The bulbs on your FW light will need to be changed or a 10k, 14k or 20k to keep the algae down. Also use RO water from the start.