Juvenile clowns can also go directly to being female (never become male) if no other clownfish are present. Probably not something likely to happen in nature, since a clown is not likely to find itself alone. However, it is pretty common in captivity. Anyone who buys one juvenile clownfish for their tank is guaranteed to have a female within a short time.
As for pairing, what worked for me was introducing one juvenile clownfish to my tank in July last year, then introducing the second juvenile in September. The clownfish were Tomatoes, which are just about as mean as Maroons, but the first clown had just enough time to get big, before the second was introduced, without being alone for so long that she got nasty. They got to know each other immediately after the second clown was introduced to the tank, spending the next couple of weeks doing little epileptic seizure dances. On January 1st of this year, they spawned for the first time, and have spawned every few weeks since.
Here's a picture of them, shortly after they paired up, showing the relative size:
BTAs are the best host for both Tomatoes and Maroons. My pair of Tomatoes have three BTAs to chose from (but they are in a 180g tank).
