Sixlines tend not to mess with polychaete worms that much. The reason is usually because most of the worms are out when the fish have already went under a rock for the night. That being said, if they do go for them, they might be hanging out under a clams shell.
I don't have much experience with pyramid snails and sixlines, but I do know a couple people that do. Most of them have had a sixline get a pyramid snail population under control within 72 hours. Similar to peppermint shrimp, they will eat A LOT of crustaceans that are small enough for them to get in their mouth and crush (so tiny ones).
I have kept sixlines with clams dozens of times though and never once had a problem. They can be territorial and aggressive towards other fish, but usually not larger inverts.