1) all freshwater crabs require a land base area, crabs generally require 40% land and 60% water, the common crab: the fiddler crab which is brackish does require both land and brackish water. all crabs are aggressive towards anything in the tank.
2)prawns or crayfish are aggressive and should only be kept by themselves or with high tier fish such as hatchets. crayfish will attempt to eat or nip low tier fish such as corydoras since they are basically always on the gravel. they will vigorously defend their territory. since the black ghost knife fish is a low tier fish i highly suggest not buying any crayfish. as with shrimps i will not suggest buying oriental shrimps such as cherrys, red or etc.... as they will be quickly eaten by the black ghost knife fish. if you do want to buy shrimp just due to the fact you want them i would recommend ghost shrimp, as they are feeder shrimp but 20% of the ones you buy will survive not only from the fish but also from the temp. ghost shrimp are generally kept in cold water tanks, so high acclimation is required with these guys.
3) there are 2 common freshwater frogs.
-african clawed frog: highly aggressive and will attempt to eat any fish it can put in its mouth. get about 4-6 inches
-african dwarf frog: very peacefull community pet but will be easy food for the black ghost knife fish as frogs are bottom tier and aren't the fastest reactors. not forgetting the fact they are fairly hard to feed as 98% of the african dwarf frogs will not accept frog pellets. i had to construct a special feeding area that is only obtainable by african dwarf frogs and fed them frozen blood worms, which they love. but they are also very very slow eaters so usually their food is stolen without the feeder.
all in all i do not suggest any of you choices, except ghost shrimp.
due to the fact you have a black ghost knife fish in a tank that is the minimum amount a space one can live in, i would generally leave it by its self. even the fact that they have the ease to grow to over 1.5 feet even in captivity.