Good call on the malaysian trumpet snails. Your local Petco or Petsmart will likely give you dozens for free. They know them as pest snails.MTSs would be better or if you have a large enough tank, corys will eat the dropped food. Barbatus corys like the water a little cooler then most corys and I can see them being a good match for goldfish even so I have never kept them together. Something you might want to check into. Good tank mates for fancy goldfish are dojo loaches or white cloud mountain minnows. Both would go after food the goldfish drop. But my goldfish always cleaned their gravel themselves. They might drop some food but later they will pick up the rocks of the substrate and suck on them getting whatever they missed the first time around. I personally would only add MTSs and nothing else.
Clams would not be on my list of critters to help keep a tank clean.
Are they bigger than Amanos? The Amanos didn't make it...You could get a couple of bamboo shrimp. They are filter shrimp and take small particles that float in the water column. They should be large enough to be safe from most types of goldfish unless you have comets or commons.