Personally I think the safest way to bond is slowly, I do not agree with the smoosh method at all it's seems cruel and chins have gotten seriously injured and even killed doing that. I realize some breeders swear by it, but maybe they somehow do it differently, but even if they don't get injured or killed the idea of intentionally traumatizing the chins so they hopefully bond over a shared trauma just seems cruel, but maybe that's just me. (my opinion and those of experienced pet owners tends to differ from breeders). The only benefit I see with it is you will know that day if they will bond or not rather then spending weeks or months trying to slowly bond them only for them to not end up bonding. The cage in cage method is a bit safer and less cruel, but still risks the chins getting noses and toes bitten.
So with the slow way, start with keeping the new chin quarantined a different room for 30 days, that allows any illness to show up as well as for the new chin to settle in to the new home so you know what behavior is normal for that chin. Then after the quarantine you can move both cages into the same room and slowly move them side by side as they get use to each other. Once they are good side by side then you can try short playtime together, like 5 minutes, in as neutral an area as you can (if you can't use a different area at least use new items in the area that neither has used so it's not the territory of one or the other) and increase the time from there if they get along. Once they are getting along during playtime, eating side by side, taking a dust bath together, etc, then you can try short periods of them being in each other's cages, then in the same cage together. This slow bonding allows them to from a bond over time similar to how you build a bond with your chin.
As to age, it doesn't seem to matter as much other then the size difference, personally I would wait until they are both close to the same size or at least 4-6 months old before having them in the same cage. You can introduce them during playtime but you don't want one over powering the other when you aren't right there to separate if needed.
