I used to breed Neon Red Lobsters/Crayfish (Procambarus clarkii) and so I had quite a lot on my hands and had spread some out into other tanks.
Now this is a different species of crayfish so my experience may be completely different than those keeping Marmorkrebs. I am not familiar with Marmorkrebs and how their temperament is.
But just wanted to mention my experience with the crays I have kept (Procambarus clarkii). I've had many in community tanks, they multiplied so fast, the community tanks of fish, practically became crayfish tanks with some fish. I've had them with corydoras, guppies, swordtails, plecos, arowana, young cichlids, bala sharks, and clown loaches (different tanks). With this keeping these crays for a couple years, I've only had maybe three incidents where a fish would get cut by the claws of a cray (they recover) and only maybe three times a cray had caught a guppy and ate it. Remember these were crowded tanks with many crays .Yes the crays would challenge each other, sometimes with limbs being lost, but regrown on the next molt, when actually jam packed with crays, fights wouldn't happen. These crays wouldn't actively hunt any fish, even at night. Though sometimes, out of boredom it would seem, some crays would like to sit on top of decor, raising their claws, "fishing" for some fish (hoping a guppy would swim into it's grasp). Some would be sometimes raise their claws at a corydora if it was in their path, but many times, all the fish could move peacefully past each other. The fish were not living in constant fear. None of the crays dared to harm or challenge any larger fish, such as the pleco, arowana, cichlids or clown loaches (bala sharks were only 3-4" at the time), they might raise their claws as intimidation, but the large fish had no fear and the crays wouldn't pinch even if the fish were to swim right into it.
Just my experience with that particular cray (Procambarus clarkii). I know nothing about self cloning crays (Marmorkrebs) in a community tank. Even smaller species of cray/prawns can be even more aggressive than larger species. Best look up first hand experiences to see how their temperament is with fish and other crays. I believe there are a species of dwarf (staying under 2") self cloning crays (used to see them on eBay, Amazon, Aquabid all the time), similar to CPO Mexican Dwarf Crays, but with varying speckled colors.
For the sake of the other fish though, I too would suggest skipping on the crays and just sticking with fish, but if you want the crays, you could set up another tank just for them. If you just liked the idea of a large invert like that, you can look into Singapore/Flower/Bamboo/Wood shrimp and Giant Blue Wood/Vampire/Viper shrimp. They get a decent size and are completely peaceful/harmless. They are filter feeders, with hair-like "fan hands" (not claws) that catch food particles from the water.
