To the shrimp, first off they weren't realy planned for this tank setup, i tried to get them out, but it was impossible to get all, i sold about 750 to some guys localy, they bred like crazy.
To your question, they leave the adults mostly alone, but eat the baby shrimp. Since my tank is heavily planted and has lots of caves, they still multiply.
To your question about the number which works with them so they don't beat each other up, I started with 12 and it worked okey but wasn't perfect, I stocked them up to 24 and their behaviour changed to a more balanced behaviour in regards to chasing each other around also they became more interactable, they came to front when I was sitting in front of the tank and they sometimes even followed my hand around. With your tank size and 50 of them I think you'll be fine.