For me, I tend to not focus on labels when it comes to writing characters with mental disorders. I find that picking a label is perhaps the opposite thing to do, and instead, it feels much more natural to pick out traits and just write them. The more you try to stick to a label of a certain disorder, I think it takes away the potential for character depth, and the character begins to be characterized more by their disorder than by who they are. For example, I've got three characters currently with some form of mental disorder -- now, I could label each of them like this:
Jade - Major depressive disorder, PTSD
Édouard - Narcissistic personality disorder
Jaina - Mild Psychosis, Antisocial Personality Disorder, Histrionic personality disorder, Mild Dissassociative Disorder 

However, I find that it's much more rich and intriguing to define my characters by their traits, rather than their illness. They then get profiles instead that sound like this: (I put them in hiders since I just pulled the descriptions from my CS tab):
[hider=Jade]As much as she hates being defined by her depression, it influences most of the choices she makes. Her mind, in her opinion, is being ruled over by some demonic entity, causing her to see the worst in each and every situation. As a private eye, that type of negativity does come with the job -- but it also causes it to be difficult for her to make friends. She craves companionship, but her own assurance that nothing good can ever happen prevents her from attaining it.

The very few people that Jade befriends can experience a fiercely caring relationship. She'll do anything for them, no matter the cost. And of course, she happens to want to find things out, enjoying the possibility that the unknown prevents. She tends to throw herself into her work, in order to escape the feelings that cause her extensive pain. She is also skilled in the art of sass. [/hider]
[hider=Édouard]While some may describe Édouard as being a French Paris Hilton with mafia connections, that doesn't do him justice. Certainly, he has the vanity and needs of one who grew up accustomed to riches, and the worth ethic of a person who never had to work. From the moment he was born, everything was handed to Édouard on a silver platter, and while this did cause him to become a bit pretentious, it also sheltered him. There's an innocence about him that some may find charming, as while he's an adult, he still has a bit of the mentality of a child. He's old, but he hasn't really grown. 

This causes him to be a bit reckless, always wanting to go out and do something interesting. Édouard is constantly reevaluating himself and where he is, hoping to find something to take away the boredom. Anything that he deems to be work is thrown out, but if he thinks of it as an experience or an adventure, his attitude changes. Too innocent and kind to run the mafia, Édouard has been forced to grow up and mature from the apocalypse, as he's survived without a household full of servants to take care of his every need. In a way, he's like a Trust Fund Baby leaving home for the first time, and interacting with the real world. [/hider]
[hider=Jaina]Initially, Jaina was a quiet and sweet young girl, always seeing the best in everyone and trying her very best. She could never hurt a fly. If you went into a dictionary and looked up the definition of Happy Go Lucky, Jaina's picture would have been the only definition needed. Of course, this was largely due to the relatively sheltered life she had lived. Until her force sensitive brother began experimenting with her head, of course. This slowly but surely drove Jaina insane, to the extent that she always seems like this little cupcake of sunshine, rainbows, and happiness. Hardly anything could bring her down.

Of course, her psychosis manifests itself in other ways. Jaina lacks understanding of several concepts, the most concerning being life and death. It's a miracle she grasps the notion of object permanence still. To her, there's hardly anything to be afraid of in the world. Death is all an illusion, just something that others stress out about. Even when witnessing death or killing someone herself, Jaina is as unfazed as can be. There's a gigantic disconnect in her brain. This causes her to be fearless and confident, but also rash, reckless, and stupid. She's like a gambler, only she's betting her own life with each and every decision she makes. [/hider]
Sure, I do mention some of their mental diagnoses in these descriptions, but the descriptions don't revolve around those various disorders. Myself, for example, I suffer from major depressive disorder and general anxiety disorder, and I'm a recovering self harm addict.  I can draw on these experiences a lot, especially now that I'm recovering from all of that trauma. But I don't let myself be defined by them, even if depression and anxiety can color my thoughts and actions. At the end of the day, I'm defined by how I act and who I am--and that's more than just my mental disorders. 

I think if you do things that way, you've more room for character development. Any character can develop, some will just take more time than others. No person is going to be a textbook sociopath or a textbook psychopath--those aren't real. No psychopath or sociopath is going to tick off each and every little box. I usually add to that by having some sort of event that caused my characters to be the way they are. Jade, for example, was sexually abused by a genius father. Édouard was neglected by his family and could only enjoy material things growing up. Jaina was essentially given electroshock therapy from her brother. Everyone is the way they are for a reason, and I think with that in mind, any character can get development. :)

That's my two cents on this matter. ;)