Woah I read the manga but I didn't see the captain cloak on isshin, lol. I also don't remember him mentioning he is of captain level. Bad memory + other mangas I read = no!. Anyway, I think isshin was a captain but he left cos maybe then he had kids. So far we know no other captain who has kids. Could be a reason who knows. Soul society prolly allowed him to leave but remain a replacement shinigami if danger should arise. Now I can't wait to hear his story.