I have a 15 gallon tank too.
You shouldn't add too many more fish. With good filtration you could probably get away with one more that is of a similar size to your clowns, but most people will suggest stopping at two or -- if you must -- adding something small like a clown goby.
With good lighting and stable water parameters, you have lots of options for keeping a variety of corals in a 15 gallon. It all depends on the type of 'look' you're going for and the extent to which you're willing to do maintenance!!
I like what people refer to as a "bonsai" look. Basically, quite a lot of coral in a small place. This brings with it certain challenges, like minimising chemical warfare, stopping them from stinging each other, and keeping on top of each coral's unique needs.
My tank is only about five months old. I have a mixture of soft corals, LPS, and SPS. They're all doing very well, but boy do I put the work in!!