I tried culturing gammarids and i failed miserably...
Among the species of fresh water shrimp commonly available, most sources say red cherry are the easiest. I acquired a group of 25 a couple of months ago and now i have LOTS and LOTS of new shrimp. So far i´ve had two generations. They grow very fast if conditions are ideal and food is plentiful, and start breeding rather young.
As long as you have decent water conditions, all they need is a cycled tank java moss and bit of food every now and then to explode in numbers.