We thought about using a longer name that is shortened to Ari but decided quite a long time ago that we didn't want to do that - if we decided on Ari it would just be Ari.
I have to say that I'm with Dad Again on this point. I'm not a big fan of people wanting/liking a short name then naming their baby a longer name for the "formality" of it... iykwim?

I really don't like Hillary... so I would go with Ari. Maybe you could spell it a bit differently so that when people see it they think "girl"?

Some ideas:

Ahree
Ahrea
Aree
Arie
Ahrie
Ahry
Ary

Once again (IF IT WAS ME), I would go for a spelling with either H or Y as it looks more feminine...
this is just my opinion only! I know some will not agree with me on this!