Welcome Triggermagee. Good question. I advise being careful about beloved or larger than life historical figures, especially if what you propose turns a hero timid etc.
Letters are great places to get information like you've mentioned, and I suspect the author found some type of documentation on the sound of Custer's voice. A private may have written it to a mother or relative after Custer disciplined him, which is why it's nice to have more than one source.
I've found some startling character-type info in a letter and I'm researching it to use in a future story. What I plan to do is have my main fictional character react to the character flaw I discovered, or have a moment of private amusement, or joke about it with a friend. etc But, I'm not making it up, as I have it in a document from a woman present at the event. But I'm searching for a second source.
My thoughts on the subject are: You can't envision and write something negative about famous people without documentation to back it up. However, you can do what you want with a fictional character's thoughts as he relates to said historical person-re:my comment about the private above.
Good Luck!