Those combinations won't work in the long run. Oscars (at least the ones I've seen and my own) loooooooove crayfish. But cichlids in general, seem to be pretty good at rooting out crayfish and picking them apart (size pending) and/or waiting for the crayfish to molt and then doing so.
And on the flip side, crayfish can pretty adept at catching larger, awkwardly shaped or fast food if they are a species that's carnivorous or hunts. For example:
This lil' Procambarus clarkii of mine ate a whole group of Silver Dollars, one by one.
But anyway, here lies another problem with picking up a crayfish, you need to know what species you are getting first. Size and tank requirement issues aside (i.e. since you have cichlids, your pH may be too low/too high for crays), some species require diets that are more high in vegetable matter than meat matter, so you're scraps wouldn't be a good diet in the long run. And most crayfish (in general) will require a very varied, mixed diet and meat scraps won't cut it. Crayfish need nutrients found in plant and meat matter, and should have additional supplements for things like Calcium.
That aside, you'll need to take into consideration a species temperament. Some are highly aggressive and will kill off tankmates (either from hunting or just because), and your fish aren't very large so that should be a concern.
Anyway, without going into a much longer discussion, perhaps try some catfish? I have a bunch of different "hoplos", aside from their regular foods they love to suck up the scraps the other fish don't eat.
