I've never had an anemone, but everyone I see talk about them always say that a 10 gal is too small for one.
As for lights for a 10 gal, I'm not sure why you're looking for a 12 inch light. I believe a 10 gal tank is 20 inches long and only 8 or 10 inches front to back.
If you really want to give it a try, you could look at one of those clip on MH lights or a pendant.