Wrong way 'round
As I understand it, all clowns are of undetermined sex initially - they can become either male or female. In the wild they may live in a group with a dominant pair, all the 'subordinates' remain unsexed. If the female dies, her male mate becomes a female and the next-in-line unsexed clown becomes her mate. If you buy a pair of small clowns together, over time you should end up with a male-female pair with the female being the larger, more tempermental of the two.