Crawfish, when mature, can't differentiate between 'small fish' and 'big fish', it's all food to them.. and so eventually you'll find fish missing chunks of underbelly.. wish I could tell you different, but I'm pretty sure no one here has ever seen an LFS holding fish and mature Crawfish together.. and if you have, call P.E.T.A. !!
a LFS near my house were housing 4 inch crayfish in a tank full of AFRICAN CICHLIDS. I dont think they lasted long. The cichlid is one of the bigger enemies of crayfish. They will constantly attack them.
Oh I think I might be getting a crayfish. I had one before for like three years before it died. I also might make the tank half water half land for a salamander. I have done this before as well and one time the crayfish cralled out and was snapping at the salamander lol poor thing fortunatly the crawfish got bored and went back into the water.
If you have a tall enough tank, crayfish will work with top-dwelling fish. Crayfish don't like leaving the bottom to feed. Large, peaceful fish tend to also be okay, just as long as the crayfish you're getting isn't too big. Some only get to be two inches or so, others get to be five or six inches.
With crabs, it depends on the type of crab. I saw a marine tank recently that was literally crawling with hermit crabs and had a couple of bottom feeders, they didn't bother each other at all.