Should the mini Mushroom exits in the castle levels count as secret exits? It doesn't seem like it counts as the rules says they're 16 secret exits although there are 18 if you include mini mushroom secret exits in the castle levels.

No, they do not count. They aren't technically secret exits because there are no red flags to symbolize it. However, in order to get actual secret exits in worlds 4 and 7, its required to defeat the both of those bosses as mini mario anyway, thus not really making much of a difference in the long run.

Doesn't the world 4 cannon take you to world 7?

Oh yeah it does. Guess that just slipped my mind for some reason lmao. The fact still stands though, the mini castle exits don’t count as secret exits.