Is my 55 gallon tank properly stocked or overstocked?

Yes, you have plenty room to add more, I would add 3 more otos and at least 5 or more tetras to complete the school like the other answerer has mentioned schooling fish like tetras, enjoy bigger schools, and they do look cool in bigger schools so thats a plus. A dwarf gourami would be fine, I love my neon blue I have him with my guppies and he hasn't shown any agression toward them.