and so today we're going to make this clear and simple by starting off with the TL;DR - eLiquid is eLiquid. You are not going to physically damage anything by using nic salts in a device or coil marketed towards freebase or vice versa. Simply pay attention to the nicotine levels so you don't get sick. Use whatever satisfies you.
It's true, you can use any juice or eLiquid, with either type of nicotine, in nearly every tank, pod, dripper, or whatever device ever made. The confusion stems from the different measure used for nicotine levels: salt nicotine contains benzoic acid to neutralize the pH level, basically reducing the throat hit to nothing, until you get up in the high mg/ml ranges.
What exactly does that mean? It means you can use low powered devices such as pods or mouth to lung RTAs to deliver a high nicotine level per puff, without destroying your lungs or breathing in pure pain. 50mg freebase would make you cough so hard you might pass out, either from lack of oxygen or overload on nicotine. Salt nicotine also hits your brain faster, delivering satisfaction quicker, which makes it easier for people who are freshly quitting smoking to make the switch.
So why are there warnings on some salt nic bottles about sub-ohming? You wouldn't want to blast 50mg/ml salts through your profile RDA at 70 Watts, because you would be blitzed to the moon from the nicotine. That's it. It's not going to break anything like some people seem to think. Can you run your 50mg salt through that 0.6ohm Nord coil? Sure. It's a low-power pod device.
In summation; yes, you can run your 3mg freebase in your Orion or Nord. Yes, you can use 6mg salt in your sub-ohm device if you want. Yes, you can alternate between salts and freebase in your Nautilus. That's all!
What do you prefer, nic salts or freebase? Do you mix it up across your devices or preferred flavors of eLiquids? Is still really as confusing as people make it out to be?