Right. It seems to be the case that introverts are more prone to manifest depression and extroverts to manifest mania, as a neurotic vector of that innate energy. But when you have crossover, like an SeTi-Ti or SeTi-Ni then you do get a mixing of traits. In enneagram terms they may be type 7 yet also a 4, and alternate between those strategies in a bipolar sort of way.
This is still a tentative idea for me though. But in general, introverted subtypes = 4/5/1/9 , extroverted subtypes = 3/8/7/6 (using enneagram only as a descriptive system, not definitive one)