I wouldn't add crayfish to a tank of smaller fish if I were you. Crayfish will eat anything they can catch and that includes fish. Larger convicts might be OK, but I would avoid any smaller ones. Convicts can be feisty themselves as they swim by, but if the crayfish gets the drop on them as they swim by, it can hurt them. Those pincers are strong.
Crayfish ARE scavengers, you'd be hard pressed to find a better one. They will quickly devour anything they find laying on the bottom of the tank, including any dead fish. But please keep in mind that they are also opportunists. They will eat whatever they can catch. They won't chase anything down, but they will grab whatever gets within reach... very quickly.
They may do OK for awhile in a tank with smaller fish, but sooner or later they will probably end up eating one. They won't attack larger fish, but any smaller ones are fair game.
Shrimp are a better choice for a tank of smaller fish, and they are pretty good scavengers as well. But the shrimp themselves can be in danger from any larger or more aggressive fish. They are a natural food for many of them.