Many supposedly "freshwater" crabs available in the aquarium trade are either best suited to brackish salinity or require an emergent haul-out platform. As for crayfish, most have the potential to kill or maim all of your projected tank tenants (in addition to uprooting plants) - "dwarf" species would, conversely, likely be consumed themselves. However, there are some potentially compatible freshwater shrimp you should investigate - see this link (be sure to visit all eight pages) to determine what species you have interest in.