Clare_N
2, Pay attention to the canon character's sexual orientation. If they are regularly seen flirting with members of the opposite sex, they are not going to suddenly become gay.
You know, as far as most fanfiction writers are concerned, a characters orientation within the series is more of a suggestion then a fact. It's called slash or yaoi and it doesn't discriminate against using canon character or originals, though it's more common to use just canon characters for such stories. It kind of makes this a moot point. Otherwise those are some good guidelines you have there.
Moving along.
I don't have an problem with the idea of using original characters in fanfiction. I don't care if they break canon pairings either, so long as their presence makes sense. If they break the basic rules of the universe they are in then I don't like them.
Strictly speaking, most of the time OC's don't fit the universe they are written into and become the dreaded Mary Sue (uh oh, I used the M word). These usually have similar summaries so I can pick them out and avoid them. Sorry to say Rabidpixie, but long lost sisters are on the avoid list and Draco is very popular with the fangirls, so that doesn't clear her of suspicion.
Long lost siblings, childhood friends that aren't even hinted at in the series, and transfer students are all things that turn me away. Otherwise, I have seen good, well developed OC's in fanfic. Some even had superpowers, those stories are just really hard to find.
Of course, my opinion is mostly biased since I regularly use multiple OC's in what fanfiction I do write. The one I'm writing now is even breaking a canon pairing. No superpowers though.