Fallout 3's PA is a downgraded verison of Advanced Power Armor MKI and MKII(And the same type as the Texas BA Armor, But they arn't canon i believe..)
I'm pretty sure that was never stated, at least in the default game. If this statement has any basis in any official information, provided in game or outside of the game. The fact that the design has some similarities to the Fallout Tactics power armor is likely just Bethesda trying to incorporate elements of that game into Fallout 3. After all, while Fallout Tactics may officially not be considered fully canon, that doesn't mean the developers can't take elements from it if they deem it appropriate.
In the game, a terminal in the Citedel indicates that it's Advanced Power Armor MKII. While it does look different from the armor of the same name in Fallout 2, this might be because as far as I know, the Fallout 2 MKII armor looked the same as the standard Advanced Power Armor, so it may be that Bethesda just decided the MKII variation should look different from the MKI. While the stats differ, of course, this is just gameplay mechanics and may not reflect how things actually are in the setting, especially since all power armor was weaker than its Fallout 1 and 2 counterparts in Fallout 3. What IS kind of baffling is that the Enclave power armor seemed to have weaker stats than the T-51b armor, but this may just be because the T-51b in Fallout 3 is a unique item, at least without DLC installed, whereas the Enclave power armor is reletively common once you start facing Enclave.
And it certainly seems to me like remnant power armor is supposed to be Advanced Power Armor. It looks pretty much the same, except perhaps with some added details and some alterations to the proportions in order to avoid clipping and ensure that it fits the bodies in the game. And the "remnants" reference in its name certainly seem to be Enclave remnants. It just isn't called Advanced Power Armor, and this is probably just because evidently Obsidian doesn't want to have items referred to by their actual canon names and insists on giving them generic names instead. Just look at the "plasma caster" and "plasma defender", even though it seems like they're supposed to be the plasma rifle and pistol from the original Fallout games, instead of using the canon names of those weapons, Obsidian decided to go with the names I just referred to them by.
Indeed, everyone keep calling it APA mk II and how badass the enclave is but you can kill them with bb guns....
That's gameplay mechanics and has absolutely no bearing on actual canon. Due to the way the armor system works, any weapon can theoretically kill any enemy no matter what kind of armor they have, assuming it hits them enough times, and really, this hasn't changed in New Vegas, since even if your weapon is not strong enough to pierce the enemy's armor, it still does a small amount of damage. The main difference is that whereas in Fallout 3, your armor absorbs a certain percentage of the damage done regardless of how strong the enemy's attack is, in New Vegas, the change to damage threshold means that armor absorbs a set amount of damage from an attack, mainly, this means that using weapons with a low damage per shot against armored enemies is a bad idea, and you'll be much more effective with something stronger. And I'm not actually sure if this is as good a thing as some have made it out to be, considering that it renders low damage, high fireweight weapons rather undesirable, considering that they're only useful against unarmored targets, and really, considering that there's a limit to how much stuff you can carry, it seems to me that you're better off carrying around a weapon that can be useful in as many situations as possible, which, in this case, is something with enough damage to get through even the strongest of armor, the fact that these weapons tend to have slower firing rates isn't that much of a problem if you can one hit kill most unarmored targets anyway. This is especially true in hardcoe mode since ammunition has weight and you would want to set aside some weight for food and water. Thus, you wouldn't want to carry around a different weapon for any situation you could possibly encounter. But regardless, the point of this thread is not to start a debate about the advantages and disadvantages of using damage threshold in favor of damage resistence for armor.