In kmiyc when they asked Cecil why he likes wearing dresses he only said "because Daddy (Josh) likes it and it reminds him of Papa (Chase)" but in kml sides we got more answers to that, it's because Spence finds it pretty, to Spence Pete is a "prince" while Cecil is a "princess" something something like that, he wears dresses to get Spencer's attention
About an update about adult Cecil, there's still nothing yet, every 4th gen in this verse is still kids in the stories, we don't have any mention of their second genders too

A little fun(?) fact for non novel readers, there's a reason why they immediately said that "so Josh gave birth to Pete", not just because he's an omega, but in this verse male omegas cannot get anyone pregnant
Also another fact that was cut is a line about Henry could just break/put another mark on top of Josh's mark (forgot who said lol sry) this is said in kml but lemme say it here, once alphas are marked here they cannot be marked by anyone other than the omega who marked them (edit: and can only be removed once the omega who marked them dies), but omegas can be marked by many alphas and alphas can break another alphas mark