Ohhhh man they are SOOOOO cute!!! Look at those gorgeous ear, those pretty faces, and beautiful coats! Looks like you got yourselves some gorgeous kitties there
With the litter trays, I think the number depends on how big they are and what sort of litter you use.
We have 3 cats and 2 litter trays. They are BIG litter trays, and have clumping litter which is quite deep so they have plenty of room to scratch around and be able to bury things properly. We scoop once a day, sometimes more, and I never feel like we should be providing them with more litter boxes, they seem happy with the current arrangements.
But given you have a deaf, older cat, she maybe prefer an extra litter tray away from other litter trays just to give her some privacy.
Food wise, what do you feed your Persian? How much, and how often? I think generally kitten food is high calorie, and senior food is low calorie, so it shouldn't be too bad if Chelsea eats the kittens food, but you wouldn't want the kittens eating too much of Chelsea's food - there wouldn't be enough calories in it for them.
Can't wait to get more kitten updates!