Aw he's cute! One of my first ratties' name was Remmy <3 (the darker one cuddling in the photo) thanks for taking the time and care to research about intros and for getting them the Double Critter Nation. It made a big difference for us, I wish we'd gotten it sooner.
I'd like to suggest, to please check rescues, shelters, or if rats are being abandoned on kijiji or craigslist, instead of buying from the pet shop or breeder. Most of my guys & girls (spayed) were lone rescues, introduced with the help of the advice on these forums. They became great friends. One golden hearted boy Robert Pawson had been living all alone in a small tank for 8-9 months, and very literally jumped for joy when he got to meet the other rats. For the rest of his life all he wanted to do was play and cuddle his friends. Lots of lonely ratties out there who people no longer can or want to care for, needing companionship and a good home.
For the Double Critter Nation cage, fleece blanket type coverings are awesome I think - easy to change & wash often, comfy for them, and can be upcycled into hammocks or toys, or in a pile to bury themselves into when they're too chewed up
Just fold it under the plastic of the levels, between the pans and the metal pan holders below. The metal ramps can be covered too for their lil feets especially as they get older. You can get extra shelves and ramps online, more space & more shallow to climb when they get older. Litter boxes can go in the corner, "Yesterday's News" or another dust-free recycled newspaper litter is healthiest for their weak respiratory systems. I don't know if you guys have Uhaul there but the recycled denim fibre type moving blankets make excellent underpadding. Try different types of hammocks, they sometimes have different preferences but usually like the honeycomb and pocket/blanket styles, soft hiding cubes, plus small brown cardboard box houses to hide in and chew up.
DCN is awesome too if you've got a barricaded rat-safe section of your place they have access to play in - there are external ramps so they can go in and out of the cage as they like. Not possible for every space, but they owned my living room, covered in large blankets toys and box houses everywhere. The best part of the DCN was taking it off the stand for the elderly ones who could no longer really go up ramps, having access without having to climb to get back into their pad. Just a very shallow lip of the cage base smoothed over with material & a shallow incline.
Lots of DCN setup ideas on these forums. The setup here didn't have too much climbing stuff in it, and the hammocks were set to be more accessible as they were older by the time this pic was taken. Not in this version, but their favourite thing in the world turned out to be an old fitted cotton sheet piled up to make caves in a corner. Good luck