I know a guy named
Tristan and a girl named
Trista. I've never heard of a girl
Tristan until this post. To me it is a masculine name. I don't know if I really have any rules about usage. Generally, I prefer names like
Meredith,
Courtney,
Ashley, etc. on boys. I can also see those particular names on girls, I just prefer them on boys.
Tristan, however, is not one that I can see on a girl. I would suggest
Trista or Tristana.