I have a 20g, and I have:
1 true percula clownfish*
1 ocellaris (false percula) clownfish*
1 pearly jawfish*
For the clownfish, I wouldn't keep 2 different kinds in there. I only did it because my tank was a second hand, and my clownfish had been together forever, so they know each other. If you were to put an adult true percula and ocellaris clownfish together, you would have a HUGE fish fight.... Mine are so close they have mated, so that gives you an idea of how long they've been together.
The jawfish needs to have enough sand, for instance, my sandbed is an inch to half an inch wide, and when my jawfish burrows, he still gets to the acrylic bottom of the tank! So you need a deep enough sandbed to keep jawfish.