Take your first eevee, and ditto breed it.
Eevees
you can't unless u have 2 Eevees .
To get two Eevees in Pokemon silver catch a Ditto and put it in the Day Care with the Eevee you get from Bill next to the Game Corner in Goldenrod City
Girl Eevees are rare. Because more than 95% of the time boy Eevees hatch instead of Girls. Out of my overall 60 Eevees, 55 are boys. But it is possible.
It will be luck because i have 33 Eevees and 27 of them are boys. Boy Eevees are more common.
trade
No, unfortunately, you can only get more eevees in crystal version is to breed one.
Yes. All eevees can matewith all other eevees in diamond, pearl, and platinum.
umbreon
Yes they are and in Evolution too.
Breed with a Ditto.
Get a ditto and a leafeon and you will breed eevees. Then evolve them with a leaf stone. You can breed eevees with any evolved forms. It is not possible to hatch eggs as leafeons.