OK, I searched the wiki but for the first time, I couldn't get a satisfying answer.

Factions

The Follower of the Apocalypse asked me to join them, which raised the following questions:

• If I agree, will that impact my standing with other factions?
  
• Are there other factions I can join?
  
• Can I only join ONE faction?
  
• I really want to be a NCR Soldier. Will they ever ask me to enlist?

Companions

Veronica

• I asked that in the wrong section before (DLC section lol). Is there a way to use the console to reset her dialogue? I talked to her around lvl 12 and didn't know that one sentence (against the BoS) will shut her down.

Boone

• How can I get his companion quest? He won't discuss Bitter Springs with me. Should I just take him there? (I've been traveling with Cass and ED-E)

Thank you all in advance!

For Boone you need to acquire "points" to make him talk more about bitter springs. You get these "points" by killing Legion. I can't remember all of them but I know you get a point by Killing all the Legion at Nipton as well as freeing the NCR troopers at Nipton. Eventually he'll open up and suggest going to bitter springs.

For Veronica I think you are screwed. You might try killing her and resurrecting her by use of console commands.

As for Joining the Followers I don't think it matters. But I'm not sure. Didn't even know you could join the Followers.

Thanks Razala. Yea, I helped out the Followers in the Mormon Fort and once I got Idolized status, Julie Farkas asked me if I wanted to Join the Followers. I was like uhm let me think about that... LOL
Oh well, I just do Veronica's quest the next play through.

You can join the Followers and not have any repercussions with the other factions. All that it will effect is the game-ending slideshow.

It's worth it to join too, IMO; you get a Followers lab coat which has some of the best stat boosts of any clothing item in the game (+10 to Science and Medicine), plus access to the safehouse. It's also really helpful if you're looking to recruit Arcade Gannon as a companion at some point.

Awesome, thanks Gator! Yea I read about that lab coat. Cool, I'll join them tonight!

Lol not Nipton I meant to say Nelson. Kill Legion at Nelson and save the troopers are Nelson. That should make Boone talk.

Also the lab coat. I did have it. I must have not remembered joining the followers.

LOL Yea, got ya! Yea, I did that already, went to Nelson with Boone, killed everyone there, helped the NCR soldiers, but he still won't open up. -.- All he says, when I ask him about Bitter Springs, that it's the past and he doesn't want to talk about it or something like that. Hmm, I guess I'll travel to Bitter Springs with him tonight, see if that makes him talk. I had Cass with me when I discovered Bitter Springs. LOL

You can also kill every legion in cotton"something" cove. The one south of Nelson. Completing other NCR quests should work. I think you also get some dialogue when you meet the sniper team at camp mccarran with Boone following you.