The getting raped part, most definitely happens. It doesn't have to be in the showers either. It happens in both prisons and jails alike. As for the weed, you would be amazed as to what people can sneak in there. Prisoners are actually very smart and resourceful. Have you ever watched lockup? The weapons they make are incredible for the resources they have and the ways they try to get drugs in are genious
Depends on the facility. Some have good security with multiple cameras in every cell and plenty of visibility for the guards. Not much goes on in those. Others are like Mayberry. Good luck if you get locked up in one of those.
From what I've heard, prisons develop a certain type of status quo, you don't mess with that guy, and he won't mess with you. But since jails vary, I suppose it could happen.
About the weed, it depends on the location. Some prisons don't have it, others do.