I recently did something similar with a 57litre tank. (see the photography forum for some details if your interested)
I assume your tank is cycled? Does it have any plants? The shrimp will certainly need some hiding places if you have fish.
I added 12 cherry shrimp to my tank after about 3 months of it being cycled. Be very careful if you are adding them to a tank with fish. You wouldn't want them to be a very expensive meal for the fish.