All non dwarf species of crayfish are aggressive hunters, sometimes they just don't know it.
Some are mean from birth, others are good neighbors for a time, or for their entire lives. I think that it's due to how sated their appetite is, and disposition. One P. alleni of mine was a mad crazy hunter, even if he wasn't hungry. Anything in his tank would get killed and shredded, be it plant or animal. Some of his offspring were peaceful tank mates to various fish and invertebrates, never causing a ruckus.
However, in my experience, dwarf crayfish are exponentially more placid than their larger cousins. You'll get fights and deaths if there is over crowding, insufficient hiding places for molting, or an inadequate food supply. They do make much better members of community tanks with multiple species of invertebrates and fish. However, dwarf crayfish are not as peaceful as dwarf shrimp. Dwarf crayfish do have claws that are capable of catching and killing others, they just are not very inclined to do so. Dwarf shrimp have such tiny claws that they are
completely incapable of grabbing, and holding on to, a potential meal that isn't otherwise sick, incapacitated or dead.
On an interesting tangent, dwarf crayfish are called 'dwarf' due to their body size when compared to non dwarf species, such as a Procambarus sp. Dwarf shrimp, however, are not classified as dwarf due to their body size. Rather, dwarf shrimp are labeled such by comparing their claw size proportionally to their body size. So, even though Caridina japonica (Amano shrimp) and Palaemonetes paludosus (Ghost shrimp) have similarly sized adult bodies, one is a dwarf species, the other isn't. When you look at an Amano shrimp, you see that it's legs and claws are quite stubby, like a millipede. Ghost shrimp, P. paludosus have long spider like claws and legs and are not dwarf. Then there are Macrobrachium, Prawn, or 'long arm' shrimp. They have claws that are much larger than standard shrimp, when compared to body size. Long arm shrimp can have claws that are as long as, or longer than, their body.
