Invert are good but I would suggest waiting until you have more Live Rock for them to live in and eat from. I would also suggets doing a fishless cycle prior to adding any fish. The usual method is to add a piece of cocktail shrimp fromt eh grocery store, allow it to rot. The ammonia will build up and the good bacteria in your tank will grow. This will then turn to nitrites and more bacteria will grow, ending with bacteria that produces nitrates. With 2 filters in the tank the filter media will be the primary source of bacteria for the time being. As you add more and more live rock you will want to remove the filter media and use the filters for water turn over, as the filter meida will produce too much nitrate. Nitrate isn't too bad for fish but is very hard on inverts like snails and crabs and shrimp. So test kits are very important to find out how the cycle is progressing. When the tank starts to produce nitratses that measn there is a good level os bacteria starting to form. Before adding any fish you must do a water change and make sure the nitrites and ammonia have gone to zero. Then add the fish in.
A Blue Tang is far too big for your tank, as are all the tangs. Look instead at Dwarf Angels as an alternative, but yes, a pair of clown fish should do well in there. Do your research on the varieties as they are very different, some are very aggressive, others not so much.
