Do I have to be original?
This is a more general 'RP' question but I can answer it here anyway. You can be as original or 'unoriginal' as you like. Simply put, if somebody judges how 'good' your RP is on how stereotypical or not your character is... They're probably hardly worth RP'ing with themselves. The quality of RP should, in my opinion, be determined by how well a character is portrayed rather than what that character might be (so long they're 'within the Lore'). So... I've seen VERY 'stereotypical' Forsaken played brilliantly, and I've seen several atypical characters played equally well.
The main thing is that if you want to RP as a more 'atypical' Forsaken, let's say... One working with the Tauren in Kalimdor, you should do two things:
1. Be aware of how and WHY your character has become 'different' from the more typical examples of their race. If you're aware of Forsaken culture and history as a Roleplayer, you'll be more able to pull off an 'unusual' Forsaken by considering why they're different from the established norm.
2. Don't go totally off the Lore... Sure, like I said, it's HOW a character is written, not what that character is. But, 'unusual' characters will always feel more convincing, more interesting and more 3-Dimensional if they retain some aspects that are 'typical' while being 'atypical' in another sense. In other words, don't go out thinking 'I'm going to make this character the most unusual Forsaken on all of Azeroth...' A living-loving, flower-picking, Orc-hugging, smooth-skinned, happy-go-lucky Forsaken just breaks so many things that identify them as Forsaken, that it'd be hard to pull them off as Forsaken at all. It's far more enjoyable to have a complex character that is clearly 'Forsaken' in a way (since they will have, without question, absorbed certain qualities from the shared history that I've written out) but who might also be different or unusual from other Forsaken in a way for a specific reason. So... Maybe your character IS still vengeful, is still twisted, is still rotten, but still has faith in the Light..? Just a thought. Viatrix here, for all her faith in the Shadow and the Dark Lady, is still an obese glutton who is greedy enough to collect food she can't eat and clothes she can't fit in - it's an attempt to remind her of her past life and it's one of her 'unusual' properties. But being different for the sake of being different as possible is quite silly.
Should I only know the basic Lore of my class/race?
Again... You're talking about your character, I assumed. Another really good question. and you're absolutely right, your character would certainly NOT know everything written out in this Guide. It's why I've been careful, in the history section, to point out that some Forsaken will be very ignorant of this past. It's hard to remember this in RP, but ignorance is very important when creating a character that is realistic and compelling (I, for example, can't recount the recent history of my own country IRL with total accuracy and that's with modern access to rolling news, documentaries and the internet...) Also, keep in mind that even if your character does know of these events, they will have a SUBJECTIVE view of them. Too many people just regurgitate wowpedia when talking Lore in-character, yet even knowledgeable academics IRL disagree over established history.
Hope that all helps! Thanks for asking. :)