I can tell you this.....the only anemone that will stay small enough for a 5g is the flower anemone which is a non-hosting species. Believe me when I say that the clownfish you want does not need an anemone. They do just fine without one!
Also you should know that keeping anemones is not for the beginner (I have no idea how long you have been keeping saltwater so forgive me if I am mistaking you for a newbie). Better to get your feet wet before attempting an anemone.
Even though you can start out keeping saltwater with a smaller tank, to be brutally honest, you will do much better starting out with a larger tank, say a 20g or 30g. A 5g tank will fluctuate quite a bit and if you are just starting out, you will have problems with it.
HTH
