All enneatypes concern instincts:
Heart or emotional instinct: 2, 3, 4
Head or intellectual instinct: 5, 6, 7
Gut or physical instinct: 8, 9, 1
Let's look at your comment about Fi and Te being on the same coin. Fi and Te are not always on the same coin. That statement only makes sense to me if you're talking about the coin being an IFP. We could contrast the IFP archetypes (Fi-dom self vs. Te-dom shadow) or we could contrast the default pattern of cognitive behavior for the IFP (Fi-dom vs. Te-inf). Please correct me if I'm wrong.
It might make sense to tie an Fi-dom ISFP with enneatype 9 because their secondary function is Se, but I wouldn't think any Fi-dom would fit enneatype 8 [Fi-Se combo of functional attitudes don't fit the enneatype 8 description; at all]. As I've tried to explore briefly in this post, I'm not finding a logical tie between 8 and Fi (specifically) without getting creative.