I don't entirely agree with the rule you mentioned. There must be a vowel after the ⟨ti⟩ in order to produce /ʃ/. The reason the ⟨ti⟩+V (V being a vowel) becomes /ʃ/ is palatalisation.
In simple words, when ⟨ti⟩ is followed by a vowel, the vowel becomes a palatal glide /j/ which pulls the place of articulation of the /t/ towards the hard palate (or post-alveolar region) and makes it /ʃ/.1
This change has happened historically in many words, for instance, inertia, nation, partial etc.
/sh/ used only at the beginning of any syllable after the first one as in the word 'partial'" So the first syllable here is "par" and the second is "ti".
This is incorrect and misleading. Suppose there's a word op.tim; the ⟨ti⟩ is at the beginning of a syllable after the first one, but it can never be pronounced /ʃ/ because there is no vowel sound after it.
And that's the reason why ghoti cannot be pronounced fish.
1. The ⟨ti⟩ used to represent [ts] in Latin which was simplified to [s] before those words entered English and as we know, [s] and [j] coalesce to [ʃ], that's why ⟨ti⟩+V represents /ʃ/.