You can never be too nice, is how I look at things. I think it's way more useful and resourceful if someone has empathy and is nice then the flipside of that. You still need to be assertive so you can say "yes" or "no" with confidence, in terms of standing up for yourself, you can actually choose to ignore gossipy or difficult people, it is a form of standing up for yourself, just ignore them, that way you don't even have to interact or worry about them.
the best times I had in teams or playing sports was being in teams of just genuinely nice people. nothing is better than that, also being quiet doesn't mean you are not nice, if anything it's good because you can be a listener and still join in on the laughs when you feel comfortable.
I don't think nice is the problem with SA, it think it's more being sensitive to surroundings, there is some fear of that.