Could smaller ships fire on ships of the line?

Were smaller warships expected to avoid firing on higher-rated ships unless attacked first?

No; the Ottoman plan explicitly relied on smaller vessels adding their fire through gaps in the front line.

A viewer asks whether custom prevented lower-rated ships from firing at frigates or ships of the line. The answer points back to the Ottoman-Egyptian battle plan rather than to an immunity based on rate. Smaller vessels in the second and third lines were supposed to fire through gaps behind the heavy ships. Their role was to combine enough fire to offset the front line's numerical and qualitative disadvantage.