Ennegramm compatibility in a nutshell: a given type goes well with a type pertaining to the same harmonic triad (reactive-competency-positive outlook). It seems to match my observations, more or less, although deviations are very very common.
Mostly it seems like reactives don't go well with positive-outlook types, at all. Competency with positive outlook seems fine, as well as reactive with competency, I believe.