Don't get a carpet, they are the more difficult to take care of anemones, they require a lot of light. pristine water conditions, and they grow VERY large... Too large for that aquarium..
As Thomas stated, a Tank-Raised BTA ( bubble-tip anemone ) would be the best choice.. They are hardy anemones, and beautiful at that.. You can even go with a RBTA ( rose bubble-tip anemone ) which are on the expensive side, but very beautiful.. I'd try staying away from an RBTA for now, if this is your first time, you wouldn't want 100 dollars flushing down the drain on you...
You will need to get stronger lighting for both Corals, and Anemones.. IN that small of a tank, I would only add 1 anemone.. Try searching around for some PC lighting, VHO or even MH if you have the money.
Good luck
Elvis.