I suspect the Brenda case would've occurred regardless of whether she said "hi" to everyone or not.
I doubt the girl devised a plan to get another girl raped just out of boredom - there was likely some sort of conflict going on between them.

Of course, that's probably an extreme case - most people interested in taking advantage of you aren't going to do that.
I'm not sure whether being suspicious of everyone is ideal, but then, I don't know the place.  A healthy amount of skepticism is never harmful nonetheless.

Anyway, as long as you're getting along with some of the people there, things should be fine :)