I've never heard that it's used for girls and I myself can't really say that I generelly HATE boys' names used for girls or otherwise, sometimes I love it, sometimes I loathe it.
Tristan doesn't LOOK female to me although you are partly right.
It does sound a bit female.
But nevertheless, not female enough, in my opinion.
I still like it as a boys' name and would never even think about naming a girl
Tristan.
It doesn't feel right.
I like the feminine form
Trista, though.
It's a bit, bit, bit tacky, but I like it.
Like
Usha, somehow.
So, for me, there's no general rule for when to use a male name for a girl and when to use a female name for a boy. Sometimes it fits, sometimes it doesn't.
And you should always think about the child.
Will it be teased a lot with the name?
Although I can't state this in general, I think I'm more into 'traditional' unisex names that have been used for both sexes for a pretty long time.
Naming a boy
Elizabeth or such things are simply... ugly.