Tetramod, have any spawned yet? If they have, then I'm very surprised, it must be a large tank, or you simply got lucky with a peaceful pair If they haven't... wait till they do before recommending the mix.
The differences between the sexes of these young fish is subtle glolite, it won't be an easy task. The fin part is a tendancy, not a rule, and nearly impossible to differ when at a young age. Unfortunately if you simply try to add another fish to the existing female there is a strong chance one will be killed. Getting a group and allowing them to form a pair from it is ideal.