I have been breeding blue crayfish for about a year now. I mix them in my tanks, but not randomly.
The very small crayfish do well in growout tanks - they are too small to catch and eat things, but do a nice job of keeping the bottom clean. The larger ones are fine for medium sized cichlids, but I wouldn't dream of putting them in with something that has a mouth larger than them
Crayfish have their place. Interesting and GREAT scavengers. My biggest problem with them is that they eat each other. When they molt (lose there out shell in order to keep growing) they are very easy targets - it weakens them for a day or two. I started out with 8 small ones, but within 4 months was down to 3 - 2 females and a male. Both females had a clutch and them ate the male. I just sold a bunch of the little ones to the LFS, and have enough to keep them going.
Also - they are very hard on live plants. I try to grow anubias and java fern - they will shred these if not fed to their satisfaction :x
All the time I had kept the adults in with nothing but feeder guppies. The babies I put in a grow out tank with small cichlids (african and CA). I just moved the adults to my 150 where I have mature peacocks and victorian haps. Time will tell. They are trained to come out and eat the pellets when I feed the rest of the fish, so I see them regularly. The fish seem to ignore them and the crayfish are always waving their claws around like they need to prove how tough they are.