It's good to be cautious about tank size, you dont want to endanger the fish if you dont have to.
But you also have to take into account the fishes habits and lifestyle. A lawnmower for instance does pretty much nothing but perch all day long. Mine like to sit on my mag float and watch me. lol They arent a swimmy type of fish, they are perchers like alot of blennys and gobies. Liveaquaria probably has the tank size for the lawnmower at 30-35g, they can get larger than 3 inches but from what I've seen they don't like 95% of the time. I have two right now and they are both under 2 inches. The one I had for a few years started at about one inch and went to about 3.
This was just a suggestion fish as you are limited in what you can keep in a 20g so I wanted to add a fish that has a great personality, was cheap and would do double duty. (he is cool AND he eats algae). There are alot of other smaller fish you might be able to add as well, you just have to stabilize your tank and see how you do with clowns. Clowns are usually suggested for tanks 30g and up but in my opinion that is erring on the side of caution which is a good thing. The most important thing is to start slow and carefully. This is probably the most important thing in saltwater, going slow. Not a single good thing happens when you rush.
Another suggestion for fish would be a goby and shrimp pair. Like a watchman goby and pistol shrimp. They are small but it is way cool cause they will live together in a small burrow and the goby will take care of the shrimp and bring him food. They are like best friends in a tank. lol That goby wont be a swimmer either but will perch on the rocks and stay near his burrow.
So many options, so little tank space, lol