The phrase "one size fits all" is generally applied to what people wear that says the particular article fits a wide range of sizes if not everyone (e.g., watches, baseball caps, belts, etc.). There is something I noticed with the Enneagram types where people try to fit themselves or others into every possible category or concept of the types. The underlying assumption is that the categories or concepts cover the full range of possibilities and completely describe the underlying experience they're attempting to. In other words, everyone can be fit into a type or related concept. However, I just don't find that to be true.
Let's take the nine types to start. One assumption is that nine types are adequate to describe everyone and everyone is one particular type (even fictional characters). If that were true then we wouldn't need all the concepts we have that describe variations in type (e.g., wings, instincts, Levels of Development, Tritype, etc.). It's a strange thing that we're stuck with nine types because there are nine numbers on the Enneagram symbol. What if the nine types are incomplete and there really are more types than have been accounted for? What would that look like if that were true? If it were true then it would be easy for those people whose type has been described to find their type, but others might have difficulty deciding on a type if their type hasn't been described adequately. Sound familiar? People who can't find their type are often left to think that they either don't have a good grasp of the types yet or they really don't understand themselves well. Some end up finding what they think they should be finding (e.g., "I never noticed fear in myself until I discovered I was a 6 and learned more about the Enneagram", "I didn't notice anger in myself until I found that I was a 1 and then began looking back on my life", etc.). I never hear from those "in the know" of the Enneagram types say that maybe there's a gap in the Enneagram types where they just don't account for everyone or perhaps some of the types are so broad that they're really describing more than one type.
There are a lot of concepts in addition to the types themselves (e.g., wings, centers, triads, instincts, etc.). These additional concepts can be useful in understanding or elaborating upon the types. Take "wings" for instance. Wings generally describe variations of a given type (e.g., 1 with a 9-wing or 1 with a 2-wing). While some people find it useful to identify with one wing or another, some people find it more useful to identify with both wings, and others simply don't find wings useful at all in understanding "their type." The fact is that the concepts don't always fit reality. While the wing concept might work to describe one person's experience, it may not describe the experience of a different person. This is what I refer to as "one size doesn't fit all." The problem comes in when someone feels that they or other people HAVE to fit a concept (e.g., I don't know my wing, instinctual stacking, Tritype, etc. Help me find it.). If you understand the concept and it doesn't fit your experience then it may just be that the concept doesn't fit your experience.
I take two approaches to dealing with what I've been talking about.
Regarding type, we are stuck with nine types. The best way to make use of them is not to try and narrow ourselves down to only one type but to understand the types a little differently. I see the Enneagram types as describing nine types of focus that we all have available to us. We just don't use them equally. One of these types of focus influences our personality more than the others. It's not that we are a single type and limited to just that, but that we have a dominant type of focus that very often overrides the other types of focus. This dominant type of focus greatly influences how we approach life and it's seen expressed through our personality (much more so than the other types).
Regarding the additional concepts, it may be best to think of them as tools in a toolbox. If a concept fits then use it. If it doesn't then don't. The trick is learning how the concept works and when it's best used. It's not about reinterpreting reality to fit the concept. You use the tool only if it works for the situation. Remember the saying that a hammer sees everything as a nail even when it's not. You really don't want to turn everything into a nail just because you're holding a hammer.