At worst, it can feel a bit like what I want may be taken away from me. An example would be: I had a barbaric people based on RE Howard's savage,menacing Picts. With added detail from her, they became rather 'Dances with Wolves' real-world Amerindians. Have you experienced anything like this? What do you think about it? Any advice?
Are you guys co-Dming the campaign? In any case, I'm not sure exactly how (or why) generalizations fit into your game. If you mostly describe things to players in terms of things their characters experience, then there's room for both charicatures of the barbarian tribe to exist at the same time.
For example, you create an NPC barbarian leader and his group of followers as some sort of demon-possessed savages, then how could the co-DM change this? Does she have the power to change establish facts in the campaign?
And so what if she says that the barbarians like to dance and be nice to each other? *Which ones* is the question. Perhaps this is only a subgroup within the larger group of "savages" (might make a nice adventure hook) from which her NPCs originate. Ultimately, I think the problem could be solved by getting down to the specifics of which characters did what and leaving the generalizations as rumors known by the PCs (and thus capable of contradiction).
And the complexity of both types existing side-by-side might make for a better long term design anyway. I wonder if REH would have set more stories in the Pictish Wilderness, if there would have been individual characters with more of a range.