My guess is most dither fish you put in there(tetras, danios, etc) will become snacks for the parrot sooner or later. Getting a puffer would be a waste of money, he would either get eaten by the parrot or nip at the parrots fins. Assuming you were getting a freshwater puffer that is......
If you get the lobster you cant get any other bottom feeders and he will grab at fish as they swim by so you will have some casualties. Ghost shrimp will most likely get eaten by the parrot as well. The parrot is probably hiding because he is not used to his new environment yet. Test your water if you think that may be a problem.