As long as your tank is stable, you should be fine adding corals. You'll want to research your coral "want list" to make sure you're meeting that corals requirements for light and flow. As far as an anemone, I think I'd hold off. Personally, I don't feel your tank is large enough for most anemones. Even anemones that are considered easy to keep, such as BTA and RBTA, get much too large for a 20 gallon tank. Other anemone, that may not get as large, such as Sebae, are considered more difficult, usually require more lighting and more stable tank conditions. You could look at the Maxi Mini Anemones, but be aware that, as with a lot of anemone, they move around and pack a powerful sting.