Saltwater Fish Compatability-20G?

In my 20 gallon tank I currently have 2 clowns (ocellaris, they were babies when I bought them I think they are now a mated pair, they're always together and one is quite a bit bigger) a mandarin dragonet (he's been alive for over a year and he's not skinny, so there must be enough copepods, he doesn't eat frozen food or dried food, but I see him snapping at the live rock all the time). I also have about 3 red-leg hermit crabs as well as 5-10 blue leg crabs and 5 snails as a cleanup crew. I want to add one-two more colorful fish, a few more snails (they keep getting eaten, thank god they're 75 cents a piece near me) maybe a few more inverts, maybe a piece of soft coral/hard coral; anyone have any suggestions? Thanks!