Although the instinctual variants aren’t technically a ‘part’ of the Enneagram types, as they exist independently of any typology whatsoever, it seems that the specific flavour they give to each type is actually one of the most poweful indicators for compatibility.
Therefore it appears that indifferent to the Enneagram types involved in a relationship, the odds that they get along well are higher when their instinctual variants are the same or at least similar. Their shared values, their main focus point, as well as their common purpose can help build a strong connection to which both types bring a different yet enriching approach.
I also could observe that the more a couple resembled in their instinctual stackings, the closer they seemed to be to having a beautiful, mutually satisfying relationship. There are of course exceptions and people with different instinctual variants can as well have a solid, long-lasting relationship.
Still, there appears to be a correlation between compatibility and similar instinctual stackings. Let's see why that might be.
Self-preserving types share a strong desire for independence and material security that makes them understand each other well and work together towards attaining the level of material possessions that will make them both feel safe and accomplished. They will also respect each other’s need for privacy and autonomy because they both desire it to a certain extent.
Self-preserving types might frustrate the Social types because of their self-centered approach and lack of sociability. SP people tend to focus on themselves and somewhat reject others - their world is centered around their own person whereas the SO types need to function within a society and relate to its structure. Both types can feel trapped and misunderstood in this relationship.
The Self-preserving will also fustrate the Sexual types whose desire for close connection will be rejected by their need for private space and independence. SP types seek security and are unwilling to compromise it for the sake of new experience. On the other hand, the SX types will push for intensity and adventure and will end up running in circles, leaving both themselves and their SP partner drained and unwilling to continue the relationship. There is an exception to this rule though, if we take the stackings into account, the Sp/Sx types will get along decently well with the Sx/Sp.
Sexual types will connect to each other in an intense, intimate way that will build a powerful bond in a relatively short time. They will share the need for extraordinary experiences, original ideas and fascinating people, but nonetheless their main focus will remain on each other and their intimacy.
As explained earlier, Sexual types will end up scaring away the more conservative Self-preserving types with their energy and search for intense connections. The latter will feel intruded upon and they will tend to retreat from the relationship, while the SX will be disappointed and probably look in other places to find the thrill and intimacy that they crave. There is an exception to this rule though, if we take the stackings into account, the Sp/Sx types will get along decently well with the Sx/Sp.
Things will get somwehat worse with the Social type, whose main focus is on their social life and the many people that inhabit it. SX types need a deep one-on-one connection while the SO type is constantly looking outside the relatioship to define themselves through their social status, making the SX feel ignored and left aside in the favor of other people that they finally regard as outsiders.
Social types can get along well as they both define themselves and each other by the standards of the society that they belong to. They both know who they are, where they stand and where they’re heading to in relationship to their outer worlds. Choosing a partner becomes easier this way, but only as long as they belong and respond to roughly the same environment. It’s impotant for SO types to find a mate from their own social class or one that they aspire to accede to.
Social types will have trouble maintaining a stable relationship with a Sexual type who will ignore (if not even go against) social rules, traditions and issues such as class and standing.
They will also encounter difficulties with the Self-preserving types, who are far too focused on themselves and their own comfort to attend to society’s requests of them.
Taking stackings into account:
(remember please that these are not compatibility rules, but merely orientative probabilities.)
Sx/Sp more compatible with: Sx/Sp, Sx/So, Sp/Sx
Sx/So more compatible with: Sx/So, Sx/Sp
Sp/Sx more compatible with: Sp/Sx, Sx/Sp
Sp/So more compatible with: Sp/So
So/Sp more compatible with: So/Sp, So/Sx
So/Sx more compatible with: So/Sx, So/Sp