JackfM, I'm so glad you had that epiphany about why Jack felt that he and Ennis should both be in the tent, following Forest Service regulations rather than Aguirre's orders. Now, I'll try to tackle a few of your other questions.
2) Jack: You ever rodeo?
Ennis: Once in a while <...>
Jack: Are you from ranch people?
Ennis: Yeah, I was.
I don't know much about what did people do back in sixties. So would you please explain.
I see two sences here. I feel something like "To rodeo' is opposite to "To be from ranch people", look what I mean: I suppose that there were two (principal??) ways of earning money those years: the first is rodeoing and the second is ranching. Both are pretty good for making livings. So there were people who have chosen rodeo and the other who have chosen ranching. (And, of course, a lot of people who chose nor of what I've listed)
...But the further I write it, the more I doubt: Maybe Jack's second question refers to something different: "Are you from ranch people [as I am, or you're from a city]?
All I need to know is the following:
Do you rodeo? — Sometimes. — [So if you don't rodeo much], then you must be from ranch people, right?
So that part in the square brackets, is it implied or not? Oh with my f*cking poor English everything I've written above seems to sound very odd.
Poor boy who efforts to express his thoughts 
Actually, most people who rodeo are "from ranch people"... grew up on ranches. Very few rodeo folk are from the city. Most of the rodeo events evolved from tasks that ranchers have to do, such as calf roping. The calves have to be "cut" (singled out) from the herd and roped and brought in for their shots, castration, and branding. It sounds kind of awful, and it is. When Jack was peppering Ennis with questions, he was just trying to draw him out, make him talk. He was just asking random questions.
3) While you stem the rose.
Oh, my. I just have no clue how to translate it! I mean, in Russian dubbing it sounds like this: "...while you were making love up there". But it seems to be not very exact.
Don't know what is more to say. :-( I'll HAVE to translate it... Somehow. I mean, to transfer this choice of words into Russian. I think I'll ask it on our local BBM forum, or in VK communities.
This strange phrase has been understood several ways. You might try googling it to see some of the meanings. Here's the way I understand it but others may have an entirely different take. (Graphic language follows) It's a euphemism for the act of anal sex. If you think of the anus as looking a bit like a rosebud, then if you apply the penis to it, it's sort of like adding a stem to the rose. That's just my interpretation. Annie Proulx likes to throw in obscure phrases like that. She does a lot of research and probably heard it somewhere., or maybe she just made it up. So, how to translate this...does Russian have euphemisms for sex? I'm sure it does. Maybe you could choose one that refers to flowers.
5) That fire and brimstone crowd?
Why do you think Ennis answers like this?
Certain Pentacostal sects of Christianity, particularly in rural places, dwell on the Book of Revelations in the Bible which discusses the proposed second coming of Christ, where sinners will be thrown into the fires and brimstone of Hell. Ennis scoffs at this scenario but underneath his exterior the scared little child still exists.
