There is no clear correlation between socionics and enneagram, and actually a uncommon way to use for deciding subtypes.
Subtypes do probably exist, meaning there is no continuum range of a certain type but a clear devision between them. But nobody really knows how and what. The best subtype devision that I've seen is the popular one of of Medved and (i forgot the other name). The producing and accepting subtypes.
They seem to correlate a great deal to what you see in real life.
The quickest way to learn about subtypes is to know many people of 1 certain type. You'll start seeing similarities and differences between people of that type.