Welcome to the community. Usually as a general rule of thumb we say one fish per every 5 gallons of water. Obviously depending on their full grown size. If you plan on up grading in a couple years take that into consideration too. That being said I'm my opinion you are definitely not over stocked. The Tangs should be fine. As someone else said they could show aggression towards one another but most likely not as they were put into a new tank around or at the same time. That's the biggest thing with keeping multiple Tangs. You certainly can, you just want to introduce them at the same time so neither establishes the tank as it's part of the Reef and for extra success buying them small will only help. I like someone else's suggestion for a wrasse. They are beautiful and very personality having fish. Take a look at a Leopard Wrasse; one of my favorites!