Welcome to the site!
Like Snake said, you can only keep one pair of clownfish in a 30g. As for fish stock ideas...A Royal Gramma OR an orchid dottyback, you would have to choose one or the other because they fight, but both are very beautiful fish. My all time favorite is the Lemonpeel dwarf angelfish, but the flame, bi color or coral beauty dwarf angles are just as stunning in color...again only 1 angelfish per tank.
When it comes to SW fish, they are very territorial, and many fight to the death any other fish that have the same colors or body type. So needless to say, the inch per fish rule no longer applies like it does with FW fish. One type of fish to avoid at all costs unless you intend to keep very aggressive fish are the damsels. They are beautiful, but tend to get mean, and they kill everything else in the tank when they mature, and will bite you drawing blood.
If you already have your tank cycled and have two sets of clowns, if you do ...remove one pair. If you have not yet begun...use a raw shrimp to kick start your first cycle, not a live fish...and definitely not a damsel.
Also..do you have the tank set up? List your equipment and test kits along with their exact readings so we can help you if something is needed or off ...just to be sure, that way we can help you double check.
As for inverts: once you have enough algae...a CUC will be needed. (Clean Up Crew) of snails, crabs, serpent or brittle starfish (NOT green brittle). Good interesting additions are peppermint or cleaner shrimps, and feather duster fan worms to name a few.