@FarDraft I would argue that ILI are more pessimistic than IEI. EIE can be more pessimistic than IEI. Subtypes of ILI and SLE both need more optimistic partners to deal with their own pessimism (paraphrasing DarkAngelFireWolf69).

EIE

His boundless interest in people contributes to him expanding his social circle. In connection to this, may spend a lot of time interacting with people who will later disappoint him. When he is in a good mood, he is amiable, lively, gives compliments, admires abilities of others, is the soul of the company and readily helps people in solving their problems. In a bad mood, he is pensive, gloomy, pessimistic, and seeks solitude. Distrustful by nature, skeptical, critical of the actions of others. Appreciates words, but prefers concrete evidence of feelings and practical services.

http://www.the16types.info/vbulletin...-ISTj-EIE-ENFj

I don't think being pessimistic and being IEI are at all mutually exclusive. In fact, given the highly emotive nature of the IEI, I would expect nothing less than periods of dramatic pessimism and exuberant optimism. Beta is naturally more prone to dwelling in negative emotion, anyway.
Have you met another Gamma?

It isn't mutually exclusive but if someone has prolonged periods of pessimism I would wonder if they are depressed.

General pessimistic thinking would be more pronounced in ILI and EIE than IEI.