Now I realise that the Special Summoned Fish-Type Monsters would still have their effects negated and would still be unable to attack even if Superancient Deepsea King Coelacanth is no longer on the field. What I am curious about however, is how Skill Drain would affect this if you activated it after Special Summoning the Level 4 or lower Fish-Type monsters or whilst Superancient Deepsea King Coelacanth was face-down or off the field.

either way, their effects are negated; whether through Drain of Coel.

hmm good question.... but what about if they can attack? I wanna say they still can't because of the summoning clause and the fact that they are not tied to Coel so they would need something more.

Monsters summoned by Coel's effect will still be unable to attack and have their effects negated, it's irrelevant whether you activated Skill Drain after their summoned or not.