Okay, this is what I think.
I think that the Roman doesn't need to be educated, but shouldn't be a dumb-ass. He would be a good soldier who knows how to do more than kill people. "Street smarts" of the ancient battlefield if you will, esp. since no one else with have that skill. So he will be the one who knows shit like "those archers can't hit us" and so on. I don't like the idea of him being completely brutish.
Also how sure are you that a woman, even aristocratic, would be educated ca 1833, outside of piano lessons? I was taught they got about an elementary level education and then a musical instrument, but I could be wrong.
You could have Danielle speak Latin. That's not all that unbelievable. And the monk would speak Latin too, so there's that to consider.
As far as the monk, forgot his name, you could have him having known Joan of Arc personally and being really depressed she died.
This is my suggestion: try looking to see if there is a time in the hundred years' war after Joan's death where things were looking pretty bad for the French. Set him in that time, he would be desperate to "save France" and would certainly feel God was on his side. He would not know or not believe if told that France would win anyway. And knowing her personally/being a former acolyte would give him a serious personal motivation.
I don't have a lot of time right now so I can;t look it up right away, what do you think of that though.
I'm assuming this means the monk is French, anyway.
Also one thing that I wanted for Byron but you didn't say so much was that he is like really well-read, but on his own. As in, he has read all sorts of history, and so on.
He will be the "factbook" who knows the random things you might need to know.
Danielle the smart girl is very *intelligent* but won't know much outside her specialized field, whatever we decide that is.
You feel me there?