text - At what point the difference in x-height harm the readability?

For example, when choosing a pair of typefaces to combine in a single line, it is advised that:

• they are from different categories, in order to make them contrast


• they should have the same x-heights, in order to make them smoothly reading

_{(How to know which second typeface should be used, given a contextual typeface?)}

I wonder, what if the difference in x-height would be contrast enough, yet the smoothly reading requirement is still satisfied? Therefore, even if the pair come from the same category and it's a no-no, it would become insignificant or even unaffected to the viewer's eyes? How to make such thing happens?