You shouldn't add an anemone, period. They have very poor survival rates in captivity (whereas they rarely if ever die in the wild ; they literally live forever if conditions are to remain their liking), and I'd guess that 98% of all anemones offered in fish and pet stores are wild collected, so your taking some poor clownfish's anemone away (almost garanteeing something will eat the clown, if you think about it).
I'd also like to add that most clownfish seen in fish or pet stores are tank rased or bred, and have never even seen an anemone, and are just as likely to host in soft corals and some large polyped stony corals as they are to host in an anemone; my clown is occasionally seen trying to host in my tiny colony of 5 button polyps.
Corals(I suggest you start with muchrooms and other hardy soft corals), the smaller hermit crabs, snails,some starfish, shrimps ect. can all be added without increasing the bioload significantly. However, I can't tell you much about stocking or what you can put in your tank your tank if you don't give me a description of your set up (filtration, live rock, tank size, lighting, powerheads).