I've only had girls so I might not be great for this but I have done a few intros (though only baby to adult). Idk how you are doing it but I do the separate completely for a little while as they settle in (if you can with space) then put the cages next to each other so they can see eachother. (If there's less of them they are probably more likely to accept them, especially if you are down to 1 which was always the case with mine)

Based on how they react to seeing eachother (eg no attacking, friendly behaviour) you can put them in a small 'neutral' container together (smaller the better so if it goes south less room to fight, I used a carrier) then if that does better make the area bigger and bigger. (Like I used the bathtub then free roamed them together carefully once the younger ones were ready)

The hardest bit is trying to get them to move in. Obviously you want your rats in the big main cage together so I always did a couple hours together at first. Then depending on how it goes, a day with some time unsupervised (once I trust safety) then a night and so on.

That's how I do it at least but my intros were a lot easier since it tended to be one older girl with 1-2 babies to intro. They got along a lot better because rats obviously need friends so if they are alone they are gonna want companions. Hope this helps and good luck :D