The current advise about young chins under 6 months is that they should be kept in a low cage (they climb much more then adults) max of 24" but if you have a 12-18" cage that is even better since it's less chance of a bad fall. Personally I think some ledges are ok, but they need to be kept low and close together, not more then about 2" apart up/down and left/right to avoid a missed jump since kits tend to be crazy and clumsy as they grow. A fall of about a foot can seriously hurt an adult, but a fall of more then about 6 inches can seriously hurt a kit so it's best if you don't put ledges up higher then about 6" or the height of a hideout. Another thing to add about the cage, even though 1" bar spacing is fine for adults, if the kits are under 4-6 months (under about 400g) then the cage needs the bar spacing to be 1/2" or less or they can squeeze through the bars or get stuck trying. If the cage you have has 1" bars you can temporarily cover the bars with 1/2" hardware cloth using metal zipties to secure it to the cage until they are older.
Some other info about young kits in case you may have missed it or got misinformation. They should be given unlimited alfalfa based pellet only food (same food as an adult) along with unlimited timothy and alfalfa hay until they are around 6 months, and no treats until 6 months to a year. After 6 month you can start to take the alfalfa hay away, kits need the extra protein and calcium for growing, adults don't, and too much can cause issues for some adults. Also kits under 6 months should also not have play time (or at most 5-10 minutes a couple times a week), you can take them out to hold them and pet them if they like that, but they can quickly overheat and over exert themselves if allowed to run around. Also no wheel until 6 months to a year, again they can overdo it, and when you do give a wheel watch for weight loss.
Lastly just some general info about adding new chins to the herd, it's best to quarantine the new chins in not only a different cage but also in a different room for 30 days before putting them in the same room as your current chin. That time allows the new chins to settle in but also for any illness to show up before exposing your current chin. Things like ringworm, giardia, and upper respiratory infections aren't always obvious at first so it's best to observe the new additions so you don't have to risk treating all the chins. If you can't put the new ones in a different room at first, at least keep them as far apart as physically possible to lessen the risk.