i say w/ 20 gallons its going to be hard to keep that many fish. if you want an anemone i would just go with a species tank and have some live rock, the anemone and 2 clowns... possibly a decorative shrimp would be nice to have. other than that be careful with the corals because the anemone can sting them. i would add the corals after the anemone gets comfortable and finds a nice spot on your reefscape. maybe some zoas and shrooms would be cool in there. just my .02
ps. gotta hate land lords. heheh i just moved out of an apt where i had the same issue, except i kept my tank and moved out a month later. and good luck with the tank btw. let me know how it goes i would be interesting to see the tank once its running.