Which species of Macrobrachium shrimp do you have? I've been able to keep several different species of dwarf and filter feeding shrimp with mine for years. The only aggression I've noted is from one alpha male towards another who wants to be top dog. I even have one tank with M. dayanum, M. scabriculum, and M. lanchesteri, and M. kulsiense.
I keep the populations of each species to ~5 each, since it's a 55L tank. The main reason for starting it, was I wanted to know if any could interbreed. After a few months with one male and females from the other species, no eggs were fertilized. I repeated this with the all others. The M. kulsiense and M. scabriculum were no big surprise, since they're very different from the M dayanum (also considered assamenses) and lanchesteri. So once I had the comunity going, I just thought it was nice, and decided to let them live in their utopian crusecian society.