If you try to change the food quickly, that can lead to a dodgy tummy and serious poo issues.
Start with the one being used by the rescue while they settle in, same with litter. Then after they've got used to the change start to replace if you want e.g. if they have a pouch at night, give them 2/3 of the old and 1/3 of the new for a bit (opened pouches are fine for a couple of days in the fridge). Then slowly change right over during the next couple of weeks.
The thing to remember with cat food is if you feed a mix of wet and dry you can go a little cheaper on one as long as the other is good quality. My preference is moderate level wet (currently Gourmet Pearle, was Sheba before that), and a good quality kibble. They get 2 pouches a day between the 2 of them and dry ad lib, but both are adult cats who are indoor only. Plus Girlcat is a grazer whereas Tobias is a greedy bugger when it comes to wet food. Be guided by the RSPCA on the amount, but at under 6 months be prepared for a bottomless pit and err on the side of generosity.
You're unlikely to get the best quality food in a supermarket, and as a pp said Pets at Home is your best bet or a local pet shop while you transition and find out what they like. I do bulk buy mine on line using either PaH or Zooplus depending on what I need & which ones have the best deals on at that time.
Same with the litter, if you want to change that. Start with the original, then slowly mix the replacement in over time. But do remember that not all cats like the same litter - my friend with 3 cats has to have 1 tray with a different litter type as one cat will only poo in wood pellets, whereas that one's pee and everything for the other 2 cats has to be in a clumping litter. And yes one of the other cats is his litter brother so nothing to do with upbringing, he's just an awkward sod!