You will not be able to do a leopard wrasse in only a 16 unfortunately. They require a large sandbed as well as a good amount of microfauna to constantly snack on. Not really the ideal candidate for any tank under 4 feet long. Good new is there are some great wrasses that will not only do good in your tank, But eat the flatworms you have. The once I would look into are:
The Wetmorella wrasses, Any fish from this genus is gonna be great for your tank. Small, don't swim to much due to their reclusive nature, and great pest control. Only issue is they range a little more expensive. 75+ is what you are gonna be looking at
Pink streak Wrasse, This little guys has the same character and general care as the wetmorella wrasses but wont break the bank and has some interesting color patterns.
Six line wrasee and the four line wrasse. Both of theses guys are good options. But in only a 16 they will be very aggressive. Often killing everything else in the tank. Sixline is cheapest out of the ones I've listed, while the fourline is on the high end for a 3 inch fish. The fourline may be a little less mean than the sixline, But still a little brute.
I hope that you end up picking the fish that is right for you. But for your dream of a leopard I would highly recommend more experience and a much larger tank