There actually is a freshwater crab species commonly available for you. It's called the thai micro crab. I have kept them before and they are extremly interesting. However they are very small and prefer a well planted aquarium with floating plants. Other crabs do require brackish water and that can be hard to do if you are a begining aquarist. As for the crayfish, you normally only can keep them in a single specimen aquarium. They will shred plants and prey on anything they can catch. Even fast moving tetras could be prey for them at night when they slow down and sleep. If you want to devote a tank to crayfish, they look very nice. You also normally can't keep them together for they will prey on each other more or less. This is why shrimp are so popular, because they don't generally bother anything, and the amount of new shrimp morphs are quite attracting. I suggest you check out a website called elite inverts, they have a wonderful selection of invertabrates and are quite good at what they do. Hope this helped.