I think 'traditional' evil races should not be allowed in the Guardians, i.e draconians and dark elves.
However, from a purely roleplaying aspect, an evil character SHOULD be allowed to join because to all intents and purposes, noone should KNOW he is evil at heart.
I know its impossible to put this into practice, as its really easy to see who is good and evil. But I've always disliked how people say 'Whats your path?' 'Oh, I am evil' 'I am good, we cannot join.' etc.
I think, apart from elves and dark elves/dracs, your alignment should be a huge secret, and people will really have to know you and watch you to understand which way you lean.
So my comments are quite off topic actually. So I'll be quiet now
PS
I've just read all this. I know what I wanted to say, but haven't quite managed to put into words. So I'm sorry if it makes no sense!