Hm… I’m always kind of hesitant about gender/sex bend fic. When done right they are amazing to read, but it’s pretty difficult to do right–there’s a lot of tricky parts that need to be maneuvered around with sensitivity and there are points where you have to consider how much of a personality is due to sex (in that, unfortunately in society, people of different sexes are held to different standards and so develop differently). Like, how much change is too much? At what point do the characters we know and love become unrecognizable.
In the always!female!Carlos and always!male!Mal, Evie, and Jay… hm… there would be changes in how the parents treat their children in that case. For example, Maleficent and Grimhilde treat canon Mal and Evie as extensions of themselves/their second chances–essentially, their daughters are them. Whereas with Cruella, she treats canon Carlos as a (heh) dogsbody/minion.
If all three of them were to have sex bends I think that would also change–Cruella would probably raise Carlos (Carla?) to be a leader of minions instead of to be her own minion. A lot of Grimhilde’s advice to Evie is how to get a prince to marry–but if Evie is male then that means he would be the prince so Grimhilde would raise him to be as stereotypically prince-like as possible. Basically, training him to go on quests so he can win the hand of whichever princess he rescues. Maleficent and Mal’s relationship wouldn’t be too different except for how Mal is further from the duplicate image of Maleficent, so he would be even more desperate to prove himself to his mother.
All that being said, I do think that, yes, if the gang were female!Carlos, male!Mal, Evie, and Jay then the latter three would be very protective of the former. Carlos would be Evie’s “practice princess” in a way–like how children play make believe–while her lack of magic would make Mal think she were ridiculously fragile (especially in comparison to Maleficent’s overt magical presence). With Jay, since I figure female!Carlos would be less hostile than canon Mal and less beguiling than canon Evie, Jay would be more protective of female!Carlos. I mean, Jay is already pretty protective of the others in canon so it could only increase for a female!Carlos. Female!Carlos would be more hardened and cunning than her canon male counterpart, but ultimately I don’t think she would treat the others like her minions (that’s what Horace and Jasper’s kids are for); like her protectors, maybe, but not her minions.
As for shipping… hm… well, I’ve got to be faithful to Benlos–and if anything, female!Carlos would be an irresistible blend of canon!Mal and canon!Carlos for Ben. I would be absolutely fascinated by a male!Mal x Audrey ship–I’m always a sucker for the enemies to lovers trope. I also have a soft spot for Devie (aka, DougxEvie) but I’m not entirely sure how well that would translate to male!Evie x Doug… it’d be a mix of bad-boy/jock + nerd… As for Chad, well. I hate Chad. HATE. So I am pretty sure the Lost kids would be even less tolerant of his douchebaggery if it’s Carlos he tries to exploit–and anyway, female!Carlos would be less vulnerable to a handsome prince than canon!Evie.
As for the second part of your ask… there’s a difference between gender/sex bend fic and… well, I don’t know if there’s a better term for it… gender/sex fucks. That is, instead of always!female!Carlos, it would be a temporarily!female!Carlos. The latter can be funny and silly, but I’m concerned with the phrasing? Like, why do the guys at Auradon Prep suddenly realize he’s cute when he’s female? Why did they not realize it sooner (like Ben)? That goes into exploring the sexualities of random Auradon guys who I don’t particularly care about?
I don’t think Carlos would do anything if he were suddenly turned female–I think it would be mostly the other Lost kids doing stuff. Like Evie having another model who actually enjoys clothes, Jay glaring daggers at all the Auradon guys who keep trying to ask out Carlos just because he’s suddenly female, and Mal trying to figure out how to reverse it.
Maaaybe the only one I’d accept Carlos actually dating when suddenly female would be Ben (because, yes, I’m a Benlos shipper, but also he’s the only Auradon guy who showed any interest in Carlos before his sudden magical sex swap), but I’m not really sure where the plot would go from there. Unless it’s a story about gender dysphoria, in which having accidentally been turned female Carlos realizes that he actually wants to be a she. Thus all of her friends (+Ben) are her support system as she transitions (mentally, since Mal’s magic already took care of the physical part).
Uh.. but yes, sorry for all that heavy text. Here are some pictures for what I think I female Carlos may look like:
(This is Quin as photographed by James McCloud)