I was talking with mbbuna a few days ago and here is what he told me on the subject.
The smaller clams can live on light alone, they don't really need to be fed, but does not hurt.
The biggest problem with small clams is stress. They may be fine for a week or two, but the stress gets them. He said they are mainly imported from somewhere in the area of the orient. The shipping, distributor tanks, dealer tanks, causes a large amount of stress.
But, if you want to feed, the bowl method will work. Submerge a bowl in the tank, place the clam in the bowl, bring the bowl to the surface and add phyto. The clam will filter the water until it does not need anymore. Put the clam back in the tank.
Or, get a large tube that will fit around the clam. Put the tube into the water around the clam with one end out of the water. Add phyto to the tube and wait a few minutes. Remove the tube and the rest of the tank will use the phyto.
Chris should chime in here sometime and give his expert opinion why not to get a small clam.