There are several ways to introduce clownfish in hopes of getting them to pair - please understand the process will not be a sure thing no matter how you do it. While many recommend starting with two juveniles, IME I have had better luck starting with a bigger and a smaller fish from the beginning. Matching designer clownfish can be slightly more of a challenge, but still very doable. If you find a storm that you really like then it is not unreasonable to go ahead and purchase it now and then get a smaller fish later. Please realize that a clownfish left in a tank with no other clowns for a while will likely start transitioning into a female. When you add the smaller fish in the future it should be a juvenile and then it transition into the male.
In summary, not a sure thing but reasonable to try. With clowns that's about all you can ask for. Good luck!
