This is an interesting (albeit old--come on guys, it's a romance blog, you can't expect me to keep up) post by Isobel Carr on presenting villains who are members of minority groups in historical fiction. She notes that Georgette Heyer's The Grand Sophy, the villain is a Jewish moneylender. Although Heyer was apparently pretty anti-Semitic, Carr doesn't think the mere existence of the character is objectionable, since at that time and place most moneylenders were in fact Jewish, and successful moneylenders tend not to be soft and generous people.
But, writes Carr:
Where Heyer runs into problems in my opinion is in her actual on-the-page stereotypical depiction of [the moneylender] Goldhanger as a greedy, oily, and ultimately cowardly, Jew. Had he been an elegant, cool, hard-nosed businessman, I wouldn't have had the same negative reaction.
(Full disclosure: I was friends with Carr in high school, and yes, it's true, she has been doing historical reenactments her entire life.)
It's an interesting question for me because when I worked in educational publishing, I did a lot of books on Black history. So when I read or watch fiction (or fictionalized history) set in those eras, it's usually pretty annoying: Either you have stuff that's straight-up racist (Birth of a Nation); or you have a whole lot of whitewashing going on, where all the good guys are marvelously enlightened about discrimination (news flash--even abolitionists were, on the whole, incredibly racist); or you have works like Uncle Tom's Cabin where the victims of discrimination respond in an unrealistic saint-like fashion. (Like the Olaudah Equiano character in the movie Amazing Grace. This was an actual guy whose response to oppression was to, whenever possible, punch sombody out, and they turned him into this passive, weepy martyr.)
In some cases, I think people decide that it's simply too difficult to convey to a modern audience just how bad it was. For example, in the movie Amistad a woman commits suicide on a slave ship by jumping overboard. Totally unrealistic, because that kind of thing happened all the time, so the slave ships were set up so that you couldn't jump overboard. The grim truth is that slavers who weren't masters of forcible suicide prevention didn't stay in business long.
Amistad also features what was clearly the best-fed load of slaves ever to complete the Middle Passage. There's this impulse to be prescriptive to modern viewers--in that movie there was obviously a desire to portray Africans as beautiful, so they cast a bunch of underwear models as slaves. I've known many beautiful Africans, but I'm guessing they'd be a hell of a lot less beautiful after being chained to a shelf, starved, and abused for a few months--I know I would be.
This desire to be prescriptive is where you're getting this tit-for-tat, if-you-have-minority-bad-guys-you-must-have-an-equal-number-of-minority-good-guys thinking, which as Carr points out, is hard to do realistically in a historical romance (somehow your upper-class heroine is supposed have lots of friends from a group that is specifically excluded from her world).
I also think it can backfire. If beautiful underwear models can survive the Middle Passage with their looks intact, then slavery must not have been that bad, right? In a similar vein, you sometimes find the Embittered Minority Villain, who would be a good person were it not for all the discrimination they face. The problem I have with that is while it can be interpreted as Discrimination Is Bad, it can just as easily be interpreted as Members of This Group Are Dangerous and Should Probably Just Be Shipped Back to Wherever They Came From.
Maybe the problem is simply the two-dimensionality of it all. People are complicated, and their response to bigotry is equally complicated--especially if they're being isolated from another group. People like Benjamin Franklin and Lincoln Steffens who were publicly, openly, and horribly bigoted changed their minds later in life. Granville Sharp fought long and hard to end the enslavement of Africans, but he thought that Catholics should all be shot. Likewise, there were as many different responses to being enslaved as there were people enslaved: Frederick Douglass was infuriated by fellow slaves who so identified with their owners that they would argue and even fight with other slaves over who had the better one--and he took the opposite path.