Eh... It would be difficult to create rules for this kind of stuff. You could, in principle, make some convoluted system for tracking relative character sizes and flexibility and body types to determine what positions are possible, but Inno is already cutting features because their complexity is taking up too much dev time, so I don't think that's such a great idea. Plus then you'd end up having to draw arbitrary lines in the middle of a gradient, along the lines of "I can't do X, but I could if I was 1 cm shorter, why does 1 cm make such a difference". I'm inclined to agree that leaving it up to the player is the best option.