Can I wear the armor on the cover of the game? What is it called and where can I get it? A link to its page the Fallout/Vault wiki would be nice if they have one. Thanks for the help!

It's the Veteran Ranger Combat Armor and it's in the NCR Safe House. Do things for them and they'll give you the key. Yes, you can wear it.

Alternatively, just kill one of them and loot it. (But they don't show up until like lvl 20+...)

But you better be sneaky, they tote Brush Guns and AMRs.

They don't show up until you've progressed far enough in the story line.

Blasphemy. I don't think I could even loot one of them if they died by something else.

true. But damn Boone looks awesome dressed as one.
I would wear it but its a disguise

What dusters can I get I know caleb has one, but I spared him. And the woman in the thorn, but I dont want to kill her. Anyone else ?

Sorry Gabriel, they show up due to an event, soon after you hit the strip and progress the MQ forward you'll see them pop up in places like Camp golf.

One thing that surprised me however was after NV's strip events Heavy NCR troopers in PA poped up at 188.

Well.. That's even easier then, just head directly to The Strip, do some main questing, then kill one of their rangers and take the armor.
Could probably have the armor by lvl 5.

Will they still wear this armor if you trigger their appearance at such low levels? By that I mean, I read that Caesars Guards won't spawn/attack with the Fists if you're too low of a level, but I don't know if that's true either, since I'm usually lvl 18-20 by the time I go to the Fort.

Kill a Veteran Ranger at lvl 5? You have to be joking.

I had the savehouse key a lot earlier ... like lvl 14 latest *must load old savegame to confirm*

Confirmed now - had the key at lvl 13.

and Boone does look stunning in that armour!

Nope. Go the way through Sloan and avoid the deathclaws, find some loot, sell it for money to get into the strip, go get the YCS while avoiding wild-life and hostile NPC's.
After that: Cakewalk.

(This is depending on whether Veteran's show up at a certain point in the questline or if they show up at a certain point in the questline AND at a certain lvl, if a certain lvl is the case then it's not possible no.)

I'm not sure the exact event to trigger them now, I just headed with a 20+ character after talking to house;
As of yet have had no message on the radio or NCR npc's mentioning ranger vets and wetting themselves, which is the herold of their apperance.
I know at some point they appear, as my last level 16+ one encountered them at Camp golf fully equiped, I would say the trigger may be both level and quest related somewhere along the line.

You can find one with a sheriff's hat inside one of the shacks at Primm, one shack is label Beagle's and the other Sheriff.