I've had ghost shrimp in my 55 for over a year, and they are definitely reproducing, at least on a small scale.
Every few weeks I will notice a new small shrimp (1/4 inch or so) coming out to chase flake food.
I have large gravel, rocks, plants, and lots of hiding places, so I guess they just stay hid until they get to this size.
I started out with ten or so, and now I have about twice that many.
My water is very hard and very high ph, so maybe this has something to do with it.
Also, all of my other fish are small and peacefull.