you can do mixed shrimp, you just have teh appropriate kinds to mix. For instance, you can have cherry shrimp with amano and a caridina species (like malaya, crystal red, etc). My biggest concern for a tank you size is that the shrimp (neocaridina anyway) will reproduce so readily they will quickly overpopulate the tank. If you got something like amano, they dn't reproduce in fw and are a little larger than neocaridina and would do well in a tank that size.