Biomedia is anything the bacteria can colonize. This is where the old "rule" of 1-2 pounds of rock per gallon of water came from. If you don't have a lot of rock, you can add other biomedia like bio bricks, Seachem Matrix, rubble rock, etc in your sump to add surface area for the nitrifying bacteria. Without enough surface area, it can be difficult to build the necessary level of bacteria to handle a normal bioload.
As for the 2nd clown, that should be fine. Just try not to add more than 1-2 fish at a time. While you tank is "cycled", it's only cycled to the current bioload (aka fish poop and leftover food). A significant increase in these can result in ammonia levels rising until the bacteria catches up.
Let me know if that makes sense