Historical Fiction

I think that the rules for historical fiction reduce to a generalization of one of Mark Twain's nineteen rules for romantic fiction, as given in "Fenimore Cooper's Literary Offenses":

8. They require that crass stupidities shall not be played upon the reader as "the craft of the woodsman, the delicate art of the forest," by either the author or the people in the tale. But this rule is persistently violated in the Deerslayer tale.

I would omit the reference to the woodsman and the forest, and settle for the crass stupidities not being played. But even this sometimes is too much to ask.

As to particulars, one must limit "historical fiction" to the historical novel, perhaps as practiced since Sir Walter Scott. The Greek and Roman epics are full of anachronisms: Homer has no idea of the tactics of chariot warfare (not that anyone else does); Homer's warriors wear armor of different eras; Virgil has bronze-prowed ships and makes his Trojans recline at table like Romans. Shakespeare has bells tolling the hour in republican Rome; he uses the name of a tough and durable partisan warrior for his wastrel Falstaff; and nobody cares.

But in the novel, details count. I should say that rules are something like the following:

1. You have limited freedom with well-known historical events. You can write up Borodino as a draw, but not Marengo. You should not represent General Desaix as the victor of Hohenlinden, or place General Grant at Gettysburg.
2. The diction should be internally consistent. Probably it is better not to archaize too much. Definitely it is best to avoid obviously recent usages: Matthew Brady had better not take "selfies".
3. The characters' thoughts should be more or less appropriate to their era. They should not in the 1860s use the terms of 1970s psychology.
4. Technology and its products should not be in advance of the age.

Beyond that, I think that the rules are those of general fiction. Your characters must be plausible. They must say and do what the reader can imagine them saying and doing in life. To refer again to Twain,

5. They require that when the personages of a tale deal in conversation, the talk shall sound like human talk, and be talk such as human beings would be likely to talk in the given circumstances, and have a discoverable meaning, also a discoverable purpose, and a show of relevancy, and remain in the neighborhood of the subject in hand, and be interesting to the reader, and help out the tale, and stop when the people cannot think of anything more to say. But this requirement has been ignored from the beginning of the Deerslayer tale to the end of it.
11. They require that the characters in a tale shall be so clearly defined that the reader can tell beforehand what each will do in a given emergency. But in the Deerslayer tale this rule is vacated.