Oh hunni x it was only yesterday, these things do take time. Probably time you don't feel you have (esp with 2 kids on a weekend) Some cats are very vocal but as long as you separate any full on spats/attacks, and allow them space to separate themselve from each other I'm sure they'll work themselves out soon xx I've never had a MC but they are pretty big aren't they, so perhaps it's takin Zelda longer to teach him that actually in her view she's more important than him in the Cat Clan.
Do they have their own bed spaces? Are they allowed to sleep in various rooms, and choose which room/area they sleep in? Are they allowed up high anywhere in the house? How many litter trays have they got between them? Are any of them allowed outdoors? I'm only hazarding guesses but these may be things to consider.
I'd keep persevering, her nose is being put out of joint but I'm sure they'll sort their politics out, they are all very young.

One situation I could compare to is when I was a kid - my mum has always had a LOT of cats (between 10 and 20) and once we found and rescued a feral neutered black female cat of around 18 months with a tight white collar around her neck, so tight that it implied she'd been dumped as a small kitten. We brought her home and being pretty wild she was pretty scratchy if you touched her under her chin or on her feet or tummy, and initially vile to the other cats. She'd sit inside the cat flap when they were being called for dinner and start on each one swatting them, spitting and growling as they'd pile through the door, and otherwise liked to be left well alone by them. Some of them would stand up to her and there was a lot of yowling and hissing, but we never really worried about it.

Eventually she decided where "her" space was; mostly on my lap sitting on the fake coals on our electric fire, and having probably been alone for at least a year of her life never really bonded or indulged in normal playfighting with any of the other cats or kittens. I'm not sure how long it took for her to truly settle and the spats to stop, but from the day she arrived til when I left home about 8 years later she never did any real damage to any of the other cats (though my hand got scratched a few times at first) and she beame a happy, secure, loving cat - even though she was always a loner. She just had to stake her position in the house.

Good luck with the weekend