Alright, here's the deal.

There are many areas where factions have significant strongholds: NCR troops are found heavily at many forts, Caesar's Legion have a few outposts here and there, the Boomers have a huge military base, Powder Gangers have NCRCF and various outposts, etc etc.

People that are (almost) always hostile toward you usually respawn; cases in point being the Super Mutants of Black Mountain (IDK about Jacobstown), the Fiends and Jackal/Viper gang members that roam the Mojave.

Suppose I wanna jus' go ape [censored] on a particular group (say, the Boomers or NCR), do the unnamed NPCs respawn? In essence, do the Boomers with GLs and Missile Launcher's or the NCR troops with Service Rifles or the Legion guys with Machetes come back to life?

I already know (pretty much) that the smaller bases usually respawn their troops (at least where Legion and Gangers are concerned), but when I slaughtered everyone at Cottonwood (with Boone, if that makes a difference) none of the guys came back! This was upsetting and actually caused me enough anguish to quit playing the game for a small recess.

So, do they? And if so, are there any bases that don't respawn (this seems to be the case when I wiped out NCRCF single-handedly without the NCR's help)?

REALLY IMPORTANT SIDE NOTE: I may decide to get my Sneak high enough to gain the Mister Sandman perk, so do people who I murder during sleepy time come back?

Oh, and I know interior cells don't respawn, so I don't expect the insides of buildings to repopulate.

I killed everybody in the exterior of Camp Golf (including 3 Veteran Rangers and the Misfits, like Mags and Razz) few days later, I returned, everyone respawned, I literally mean everyone. The troopers, veteran rangers, even the named Misfits respawned. I was like WTF? The Misfits respawned?

But with the non respawning at Cottonwood Cove, I have experienced that and I think it is a bug or they did that intentionally in order to not bother you on your way to The Fort, to clear it out or deal with Benny or Caesar. Hostiles can be annoying sometimes.

Well if they are set to respawn, they will at Cell reset. IF it's the same as FO3, it's 3 days (in game)

So even if I Mister Sandman kill, they'll still respawn as well?

Haha, I wasn't surprised with the rest, but like you said, when the misfits re spawned... it was really weird. I just decked them out in Vet Combat Armor and mini-guns. They look good now.

Most of those NPCs will respawn. The exceptions are Cottonwood Cove and Nelson - those are scripted not to respawn, as if you complete certain quests they become either uninhabited (Cottonwood) or taken over by the NCR (Nelson). Nelson is related to the Restoring Hope quest, where the final step is taking it back from the Legion for the NCR at Forlorn Hope. Cottonwood is part of Eye for an Eye, where the last step is to wipe out the Legion there and prevent them from having a solid foothold on the western bank of the river (two ways to pull it off, the normal kill everyone method and another more entertaining one).

Interesting the Misfits respawned. I wonder if the game thinks they're essential at least up to a certain point because of their quest and that they have their own ending slide/radio announcement. I know there aren't supposed to be many 'essential' npc's in the game, tho, so it does seem odd.

It's not that they are essential, as they would not die in the first place if they were, simply fall unconscious with a system message about it. It's just a bug with some of the respawn code - I had the same thing happen with Cursor Lucullus at the Fort when I wiped it out, he had returned from the dead on my trip back to get the slave ledger for the Great Khans.