i have an anenome in a 20 gallon reef and i currently have 4 fish
i would recommend against it, as it takes up a good amount of space on the right side of my tank, and i cant keep corals there.
im thinking about taking it out, but my clowns host it and theyll be sad haha
i have a skimmer and no sump, but like i said, i wouldnt recommend an anenmoe if you want to have other corals (which you will, its a disease, and once it bites you, youre in trouble!!! lol)