What do you think would be the best way to translate it's lore into gameplay? Is a near immunity to small arms fire fair? Should it be harder to find and repair?

immune to all caliber below .50? ARE YOU FACKING KIDDING ME?! anything at .357 and above in terms of caliber is more than enough to put a dent in that kind of armor, eventually breaking through and turning the said Paladin into a Can of Tomato Soup, Chunky Version. .308 is more than enough to punch through the armor, especially with Armor Piercing Rounds. I think the better wording here you want is a lower DT Bleedthrough percentage, as it's right now at 20%. Personally I wouldn't mind if it was 10% or 5%, making fights with high DT enemies unforgiving, especially if you were carrying a small-arm into the scrap. Fragmentation explosives are possibly far deadlier to this kind of armor because of the chance of richoeting metal bits flying into the joints and cracks is pretty high up there depending how close you are to the blast, but the 10-5% DT bleedthrough would make the Power Armor more fun to play in instead of mostly ignored as it was in New Vegas.

I don't think any sort of damage immunity would be fair whatsoever in a Fallout game. Unless it's a .32 Calibre vs. anything other than bare skin than at least a little damage should be one, no matter how small. But I voted on it being the same as far as rarity goes because it was not uncommon to find in FO3 but was definitely rare in FNV, and I'd like to keep it that way

Light armour: 20% bleedthrough
Medium armour: 10% bleedthrough
Heavy armour: 5% bleedthrough
IMO

While enemies have just a solid 10% Bleedthrough, so your small arms still do damage...but at that point, you might as well carry something for the bigguns. Seems balanced too, Heavy Armor players get the best defense, but sacrifice in movement speed, along with other penalties such as weight.