1: Even some of the people who write racist fluffies also have non-racist fluffies, depicting it as a thing bad fluffies do instead of something all fluffies do. So it is rarely if ever a universal constant, even for those who write it. As others have implied, it’s usually just used as a justification to say “it’s okay to torture and murder this one because this one hates others over something as small as fur color”.
2: There is no well-defined lore or backstory, so we all rely on headcanons. The most accepted we’ve got in regards to canon is “Hasbio made them to be living MLPs, and they were supposed to be rare and expensive and pre-fixed, but an animal rights group (sometimes PETA sometimes undefined) found out and broke the fluffies out before they were actually finished or neutered”. Changing this specifically can get you a few weird looks, but anything else, from the public’s reaction to the creatures, to how wide-spread they are, to what happened to Hasbio afterward, to how Hasbio even did this, is different for every creator. I’m pretty sure one of our old classic users even cut out the animal rights group part.
There is also a story called “Fall of Cleveland” which many users treat as canon, but honestly I’ve never read the thing so it’s not canon for me.
This is why you’re sometimes told “all canons are headcanon”. All of us have different ideas for the fluffies’ size, weight, environmental danger, poop, lifespan, racism levels, blood functionality, exact intelligence, etc. The only constants is that the fluffy has to be comparatively squat, fat and horse-like, naive/stupid, and speaking in fluffspeak (just imagine uwu-speak and you basically got it). If someone complains about your fluffy not being fragile enough or stupid enough or racist enough, they’re forgetting about this and demanding others adhere to their headcanon, so you can easily ignore such a person. But if multiple users agree that something feels wrong, please be open to the idea that you might have actually strayed a bit too far. We have a very blurry and smeared line so it can be hard to tell what’s wrong, but we do have a line nonetheless.
And if you do end up straying too far - don’t worry. That’s fine, we’ve all messed up like that. So nothing to feel bad about.
3: Everyone has different reasons. Some use it as vent art, others find it funny ala Happy Tree Friends. My reason is specifically that they are cute and innocent, and get upset and cry really easy, and cute things crying/being scared makes me want to cuddle them real bad. Hurt/comfort reaction. (I mainly do moronbox content though.)
4: This has been covered pretty well.
5: A good metric is “the story should not be able to work without fluffies”. If it’s about someone picking up and adopting a fluffy, make sure said fluffy does things that can’t be done by, say, a dog. If it’s abuse, make sure it doesn’t come off as beating up a regular child. Make sure the fluffies behave like fluffies, instead of Warrior Cats characters. Etc.
But also, please remember to give the fluffy some kind of personality. We have dozens upon dozens of stories where it’s just some dude finding a yard invasion with a racist evil smarty and killing every living thing in the vicinity - but because none of the fluffies have personality, it ends up reading like a dude stomping out a bunch of dog toys. Find something to make your story unique, instead of copy-pasting someone else’s plot.
6: Fluffies have evolved into their own thing, but there’s still some people that draw MLP fluffs so they’re not completely separate either. Fluffsplosion does some adorable Pinkie Pie fluffy stuff. So if you want to use MLP, go for it! If you don’t, that’s fine too!