i dont know much about crayfish, but a bit about snails.
if you put a relatively large snail in with the cichlids it should be fine so longs as its not the same size as the fihes mouth. other than that keep them well fed so that they dont go for the snails. get snails that are not prolific like golden apple snails which grow quite big and manage algea or you will have lots of babies but this shouldnt be a problem with hungry cichlids.