I am writing a novel and trying to develop a character using the Enneagram of Personality model. Is it acceptable to combine more than one enneagram type for a single character ? I understand that a story might be deemed "acceptable" if the readers enjoy it, but as a technique of character development, does such combining of enneagram types hinder or bolster the process ?
Most stories are not psychological studies, and even those that are are not necessarily accurate. Indeed, many story characters undergo far more trauma than most ordinary people could ever psychologically endure. (What characters in you average police drama would not be invalided out with PTSD after the first season in real life?)
Instead, stories are a response to a deep psychological need in human beings. We need stories to make sense of our lives in a universe which does not seem to make much sense at all. If you take a religious view of the universe, there is indeed a overarching story to the universe and to our lives in it. If you do not, then the universe is essentially random, and so are our lives. But the idea of random lives lives in a random cosmos is psychologically intolerable to most of us, and even for the religious believe, we want constant reinforcement of the idea that the universe does indeed make sense despite the randomness of our actual lives.
Stories exist in a universe that is much neater and more logical than the universe that we actually live in, at least on the scale of ordinary human lives. Characters inhabit that more orderly and more just universe. Their psychology is therefore not the psychology of real human beings but the psychology that we want human beings to have in the more logical and just universe that we like to pretend we live in.
So, character development is not about constructing psychologically accurate personalities, but about constructing satisfactory inhabitants of story world, characters that make us feel the universe is a little less crazy, a little less indifferent, a little less unjust than it really is.
The basis of character in story is desire, and what the character is willing to do to achieve that desire. This fits a character into their story arc, which consists of repeated attempts to achieve a desire, each of which is rebuffed by some opposing force, until we reach the climax in which we discover just how far the character is willing to go, just how much they are willing to bleed, to achieve their desire. The crux of most stories, and therefore or most characters, is thus moral rather than psychological. (Whether you believe we are free actors or not, we all want to believe that we are, and most stories come down to a moral choice of some sort.)
So, while characters are no unconnected to real psychology, the shape of a character arc, and the demand of the psychology of the reader for a satisfactory story arc, are the real driving forces behind the creation of a character. No psychological theory, therefore, really gets to the heart of character creation (unless it is the psychology of the reader). It is story rules, not psychological categories, that will determine if you characters are compelling ot the reader or not.
There is no one-size-fits-all solution to writing. Every writer is different, and you have to find out what works for you. Your learn to write through experimentation. The only thing that will hinder you is if you expect following advice from anonymous users on the internet will make you a good writer.
My experience is that you know what you need to do. Trust your gut. If a combination of different enneagrams seems like it solves your current writing problem, then it probably does.