Honestly, I would wait before adding any more fish. Are you doing regualr water changes, at least 20% every two weeks? Do you keep tabs on your nitrates and check to see if you have any ammonia or initrate spikes after adding fish?
Yours is a pretty new tank and you have a pretty good fish load in it now.
There are several varieties of shrimp that will make a good addition to your tank. They do not tolerate high nitrates, though, so you want to make sure they are 12 or below.
I have 5 peppermint shrimp, one blood red shirmp and two cleaners. The cleaners are out the most. The peppermints come out to eat and when the lights go out. the blood red hangs in a cave, but I can still see him. Your coral beauty may kill a serpent star so go with something more substantial if you add a starfish. a larger brittle star would work and they are easy keepers as well.