Yes you can put fish, but make sure its something thats fast. Maybe danios would be too quick for them. You can also go with dwarf crays (found online). You can put alot of them in a 29 g, plus they are fish safe. Of course dont add any predatory fish or else the crays might be in trouble.
i got a blue crayfish about 2 yrs ago from petco. had him housed in a 30 gal with neons. though every once in a while one would disappear. probley to the cray belley. Now i have him house with 4 blue gills. they never bother him or he bother them. i also had him housed with a pearcie for a bit with no issues.
I had 2 crays (one escaped last week) in a 29 gallon with tons of red cherry shirmp, tons of fancy guppys, and their offspring, 2 cory cats and some tetras. I've only ever seen the cray eat one fish.
Keep them well fed and they should not bother the fish unless you have a fiesty crayfish.