Leo is fine on its own, but it may be wise to make it Leon, it gives it that full first name effect and you only have to add one letter. But its a beautiful name
I think Leo can stand on it's own. It's a cute name, and it'd be great for a little boy born under the astrological sign of that name. I also Leonard and Leonardo.
I prefer it as a nickname for Leonardo, and I've seen it as a nickname for Leonard. But, whichever, I don't care for it. I think of actor Leonardo DeCaprio and the astrological sign.