with that setup it will be difficult to keep an anemone in there. I would recommend just not getting one, a lionfish, pufferfish, and eel are already quite a nice mix but you'll really be shooting yourself in the foot if you add an anemone to the mix as well. 1. It might get eaten. 2. They might get eaten. and 3. It may die due to poor water quality associated with keeping so many aggressive fish together in such a small tank and then as a result everything else may die from the toxins released by the anemone when it passes. Because anemones don't just die and fall over dead. They melt. It smells terrible and it makes an enormous mess.
I would avoid an anemone.