> I am talking about the specific and highly unusual behavior of crawling inside of a much larger animal's mouth and trusting it not to eat you.

OK.

> Cleaner fish are highly intelligent[1]

Yes.

> and it appears that this intelligence is necessary

Manifestly not, at least not in general.

> for their niche:

That is an open question. Just because an unintelligent cleaner fish hasn't evolved doesn't mean it couldn't.