While it would make sense that a more populous type would tend to have more variety within it, the relationships between MBTI and enneagram and population are all fairly complex, so there may be some variation based on correlation tendency - for example 1 typically correlating to more MBTI types than 4, though both tend to be less common types. 8 seems to occur across extraverted types minus ExFJs. Undoubtedly many combinations are highly unlikely, and it is prudent to assume up front that one does not have an unusual type combination until there is sufficient evidence, if not simply for the sake of statistics then especially since newcomers to any subject are likely to generalize and misunderstand at first.
At the same time, I have yet to see a solid argument for completely ruling out the possibility of the less popular MBTI-enneagram type combinations. Because they measure two different variables - patterns of perception and judgment in cognition and strategies for ego protection, respectively - there is no obvious inherent reason why a certain quality on one plane would impact the other insofar as to make them mutually exclusive. The only way that could happen would be if cognition pattern or protection strategy ran up against one another, making it impossible for the other to successfully function. A combination that comes to mind would be ESTx and 9, but then we consider that ISFPs are often 9s and INTPs are often 9s and ENFPs lay claim to a few as well, so no discrete preference - not E, S, or T - would rule 9 out. So really we could only make that judgment if there were something about the holistic type that runs up against the enneatype (or vice versa), and I think that would be very difficult to "prove" to any satisfactory degree. Unfortunately it strikes me as one of those gray areas we cannot really address, as it would require such a huge inductive leap.
My opinion as it currently stands is the best we can do is to say "# is most correlated with A, B, and F", and encourage people to consider those probabilities as much as possible before moving on to the more unusual combinations. But then there is the question of whether there ultimately is any such thing as "true type". Some go so far as to argue type is not static, and there is substantial evidence that Big 5 traits change according to predictable patterns over time, so it is not a completely unfounded argument, though it flies in the face of the conventions of personality psychology.
So, those are my mullings on the matter. Regardless, if you can provide a watertight logical argument for the impossibility of certain types, I am interested and would be impressed.