WHat kind of lighting? DO you plan on coral?
If you are going to add coral, I wouldn't add too much more live rock, especially if you are buying frags attached to rock. Maybe just a few, small, strategically-placed pieces of rock to create a good foundation.
You may want to consider just a light dusting of aragonite to give your clean-up-crew something to play in, but I like the bare bottom too. Easy to keep clean. But with space being limited, you may want to use the floor for coral, in which case you should have a substrate.
On fish, I just added a 1/4" false perc to my 3 gal today, and I am probably going to take it back to the store tomorrow. I wanted to try it, but I believe it is too risky, with all the feeding that is required. You might get away with a clown, I agree. But watch your feeding. I would turn off all flow so nothing is swept away when you feed, and only feed enough so the fish can finish it all in 1 minute. If he is still hungry, add a little more, but be very very careful.