Moreover, the sentence on page 88 explicitly says to learn all skills of Ukobach and Slime beforehand. They learn them all at Level 7. The Angel doesn't need to be leveled up, so you can just win one from a shuffle, and the fusion will work. Ukobach is ranked higher than Slime if they're both at Level 7, so Ukobach will be the Third Persona anyways.
Sorry, but this is simply not true. I did exactly what you describe and I got the same result as the TC. I had to level the Ukobach up 1 level to get it to work.
From further experimentation, I know what the probem is and it ties in with another error on page 215. On page 215, 2nd paragraph, it says that if you have 2 personas tied for the highest level, then the third persona will be the one that has the
highest Arcana (e.g., between II and X, X would win).
However, starting from this problem and doing further experiments in game, I have learned that the "third persona" chosen between tied personas is actually the
lowest Arcana. From the example above, II would be the real winner.
What I'm also curious about is that since triangle spreads use
current levels, what happens when the "third peronsa" is tied not only for highest current level but also Arcana? (E.g., If you were to level an Ose up to 38 [Fool] and fuse it with a Black Frost Lv 38 [Fool]and a Pixie Lv 2 [Magician]). I imagine it would then look at base levels?
Edit: Scratch that last part, it doesn't matter. Either way you slice it, you would be using a Fool as the third arcana and using the other Fool for the initial fusion.