Don't feed any pieces larger than about 1/4 inch to a smaller anemone, it can cause problems.
If the anemone is getting plenty of light, a couple of times a month should be OK. You can also give them a squirt of mysis, if you feed those to your fish.
It's the anemone that hosts the clownfish (not the other way around). Some people say they've taped a picture of a clown (same species as theirs) in an anemone to the tank, and the fish have then started using the anemone. I don't know if it always works, but I guess it's worth a try.