In my longest running campaign, our first adventure was from an issue of Dragon. It set us up as all from the High Dale in FR. Researching the area enabled us to come up for reasons to have all four of the then core classes (Fighter, Mage, Cleric, & Thief). After that, we slowly added PCs that fit whatever our current location was when new players joined. As a DM, I always insisted on the PCs fitting into the campaign or at least having a logical reason for being there. As far as alignment goes, in real life people with many different alignments can be friends so why shouldn't the same go for PCs? Most people who know me would consider me NG, my wife would be CG, and my two best friends would be LN & CN respectively. We became friends as teenagers and grew into who we are now. We may disagree with each other at times and we've even got into fights with each other on occasion, but we stay friends. I see no reasons why the same couldn't be said for PCs.