The reason why he appeared to be playing with the shells is that there was still parts of the snail in the shell. He was just trying to get every little bit out.
I wouldn't say he was harrassing, he was really just hunting.
Compatiability isn't just tank size for fish, its also what fish will get along with other fish.
Triggers are very smart fish, some will blow jet streams of water into the substrate in search for food. (mainly seen from the Bluelined Triggerfish)
If you want to keep snails with your trigger, the best way is to keep your trigger full, and then again thats not a guarantee.
Hope this helps