Problem isn't necessarily your choice of fish. Its the territorial nature of a mated pair of clowns and they are jerks that will fiercely defend what they think is their turf. It sounds like your pair has already "claimed" the whole tank so I don't think your issue the choice of fish.
I have a pair of clown + 4 other fish on a 30L and they all coexist "peacefully" in the sense that everyone stays in their established turf. This will be the situation for any mix of fish + clowns in a nanno tank. If there isn't overlapping turf, any fish can work, if clown already claimed the whole tank, nothing will work.
I suggest with the next fish you add - pick one based on ur preference like color, personality, etc, "toughness against bully" is helpful but not required. When you add the new fish, change your rockscape completely. If possible try to use rock to divide the tank into new "partitions" or zones. If your clowns already is hosting an anemone or coral, move that to one of the smaller new zones. With some luck it will trick the clowns into reestablishing their turf on only the new small zone, leaving your other fish more freedom to swim around without harassment.
