What separates these two labels?
well to start, if you haven't seen it yet, here's a link explaining the ranking system.
but basic answer, the people who are knowledgeable or adept monitor what you do on the site and if they have time, will rank you. i'm told it isn't something they do too often so it's best to just ignore the label. [its like ranking spells and articles, everyone can do it, but how often do you?]
Is one easier to obtain?
you are currently no rank, if you are ranked fluffy it won't appear on your wall but other mods can see it i think. the easiest is beginner, that means one mod saw you post a lot of good information and ranked you. most people stay there for a bit and maybe move up to knowledgeable but i don't know many who have gone past that. i've been on the site for years and have been practising for 10 and i've never reached knowledgeable [but i feel novice suits me best right now so whatever]
Can a knowledgeable person have fluffy behaviour, and visa versa?
no knowledgeable that i'm aware claims to also be a giant invisible bunny or whatever, but i do know a few that believe in things one may consider fluffy like fae, but the belief is still in the realm of commonplace [like you cannot become a faerie, but they are real and you can work with them]
Can a knowledgeable person's label become fluffy, and visa versa?
like if Lark [example] was suddenly ranked down to fluffy? it could happen, but it's not likely to happen. the ranking system works by votes, lets say in theory the knowledgeable member has 10 votes for knowledgeable, even if 10 people came and voted them fluffy it would even out at beginner i think. but i believe members can change their votes, so if the 10 people who voted the user knowledgeable either deleted their account or changed their vote, then it would become a fluffy rank.
Is it worthwhile to provide information on this site, or should someone just avoid it all together for fear of being labelled as fluffy?
eh, do whatever you feel is best. there was a time where all i did was post/edit articles and spells because i'm a little bit of a neat freak and i wanted to fix stuff, on top of that people kept asking me for spells so i started posting as many as i could so i could just refer them to my spellbook. i've never typed up an article for the forums [outside of my coven anyway] but you could, it might come under scrutiny but thats just expected on an online forum where you can post and comment anything [provided its not against site rules] so you may fully believe in spirits, you might get someone who comes along and calls you a fool because they do not believe in spirits. i think posting information as well as the ranking system you should just let it roll off you. i get it can be hurtful sometimes, but just be confident you know what you know, but don't be close minded to anothers view if they're simply trying to help or join in discussion.